Microsoft Windows is the most mainstream PC operating system that ever existed next to DOS. It was developed in 1983 and released as version 1.01 in 1985. However, it did not catch on until 1992 with version 3.1, and did not become the success it is today until Windows 95 in - you guessed it - 1995, 10 years after it's introduction.

The original versions of Microsoft Windows were basically a "graphical shell" that ran on top of MS-DOS, and typically the computer did not boot to those. Windows, I believe in this early period, was more like an application looking for a purpose than some purpose looking for a GUI interface application. That's why most people ignored Windows until 1992 or later, because the majority of comptuer users were seasoned pros at using the MS-DOS CP/M derived command line., so Windows failed for it's first 2-3 versions. In the early 1990's thouggh, with the advent of the internet and "computers are the future" being a part of education Windows was seen as a way to get the computer illiterate to figure out how to use an IBM PC more easily. It meant they did not have to train future employees not hired for I.T. related roles how to use commands like "dir" "CD\" and how BAT/COM/EXE files can be run. Since standalone software could be installed, and manifest itself to Windows as a "program item" - a singnle icon in a "program group"/MS-DOS Executive/Desktop, they did not need to teach the user more than how to click on a mouse. It made things cheaper and easier (albeit slower).

Windows 95 was truly the beginning of the "modern" Windows. The familiar Start/Taskbar/System Tray layout with a wide-open Desktop above it became standard from that point on, as did the PC booting directly into Windows from Startio (prior to 95, all Windows installations booted directly into DOS, and then you typed "WIN" at the command line to kick off Windows).

Today Microsoft Windows continues with versions 10 and 11, but they owe their lineage to another Microsoft Project, a joint one with IBM known as OS/2. In 1990, IBM and Microsoft had a falling out, leading to IBM taking their half of the project to continue to develop OS/2, and Microsoft taking their half to devleop Windows NT - NEW TECHNOLOGY - with. Windows NT was produced as an Enterprise-oriented operating system starting with version NT 3.1 in 1993, and continued through version Windows 2000 (NT 5.1) in 1999, before the entire Windows Product Line went to NT in 2001 with Windows XP - with the new designations being Home, Pro, Enterprise, and Ultimate. Home aimed at home users with a a dial-up connection or broadband internet, Pro for "power users" or Enterprise class users at home and small businesses with tiny network, Enterprrise obviously being for integration into corporate networks intended to work in tandem with Windows Server, and Ultimate being a special edition for power users, gamers, and general fans of Windows with a lot of extra features, bells and whistles, and other stuff.
Windows "Eras"
I can split Windows up by era. The "early Windows" era would be versions 1.x, 2.x, and 3.0. These versions of Windows were more or less designed as an "E-Z" interface for DOS machines that might have an inexperienced user behind the screen. Today they are mostly useless as there was limited software written for them, they are quite limited in how you can configure them, and the applications contained are quite limited in their capabilities from even a standpoint of their own time period.

Windows started getting good during the "Windows 3.1x era". These were still 16-bit Graphical shells that ran on top of MS-DOS but Windows 3.1/3.11 + For Workgroups were all the first time Microsoft really put any real effort into making them a usable user-environment beyond a mere graphical shell for specialized yet castrated Windows apps and DOS applications. At the top, end, Microsoft now offered proper networking over various protocols and topologies through Windows, albeit using the underpinnings of Microsoft LAN Manager/Network Client 3.0. SVGA Graphics were availible for the first time, allowing for "Darn near photorealistic (256 color)" images on SVGA compatible PCs - and were advertised through a 256color.bmp wallpaper provided (honestly marble.bmp is still my fave). Screensavers came standard to prevent burning in your CRT monitor, various features such as APM for Laptop computers were provided as well. Windows3.1x was the first version to get on the internet, and in 1994, WIN32S allowed Windows NT and some WIndows 9x applications to work with 3.1x. The 3.1x era runs from 1992-2001, as many of these systems stayed running well past the dawn of the new millennium.

The Windows 9x era runs from 1995 with WIndows 95', through Windows 98 Second Edition in 1999, but carries on until about 2004-2005 or so. Windows 95 was a big deal at the time and a step up form 3.x by basically creating the interface we users have been using in Windows and even some Liunx distros ever since - that familiar "Taskbar" at the bottom with a "start Button" and a "System tray" hosting status icons for networking, volume, various resident applications we might need a "ticker" for, and of course the clock/calendar. This layout has been ubiqutous ever since. Windows 95 introduced us to a more "Internet Focused" O/S with more focus on gaming as well with Direct X. The ghosts of 95 still live everywhere in major civil services such as your local Police Department, Fire Department, or Hospital through the development of Active X control web apps in the 90's now causing us headaches today with Microsoft Edge and other Chrome-A-Likes. The 9x era drew to a close when the "Modern" era came in.

The Modern Era - "2001-present" - is the age of 32-bit and 64-bit NT based Microsoft WIndows, starting with Windows XP in 2001, and continuing today through Windows 11 - whith a little sub-era of Windows 8 in 2013-2015, where Microsoft tried to replace the original Windows UI with a Tablet-like start screen and "Charms" interface - initically called "Metro" and renamed "NEw Style UI" - which users hated so for Windows 10 in 2015 they went back to the regular Start Bar layout. However, I'll be largely ignoring this era of Windows (unless I get enough requests for it) - ending on Windows XP.
Windows DOS Based Versions were intended mostly to be a graphical "helper shell" for DOS at first, while later developing into their own thing starting around Windows 3.0. Windows 3.1 was the first major version of Windows that had a lasting impact. The only reason you'd want to install any of these today is in order to run old Win-16 applications on an older computer, which if that's the case, I would choose WIndows For Workgroups 3.11 as that will yield the best user experience out of the lot.
WINDOWS 1.x (1985) Windows 1.01, Windows 1.02 PC or XT class PC (8088-286) with 512K RAM or More, about 3-4MB of Disk Space, with EGA video Can't think of ANY popular applications for Windows 1.x Windows 1.x did not sell too well but it's most famous moment was the re-surfacing of a commercial on YouTube in the 00's of a hyperactive Steve Ballmer going on one of his famous rave-fests about Microsoft Windows showing the "incredible" feat of copy+paste a picture of the car from Miami Vice into a Windows Write document or somesuch on an IBM PC AT. Today this would be a subject of much riducule but back in 1985 it was amazing to see a picture of a car that looked even remotely real on a computer monitor, let alone be able to move that car between 2 different programs without some serious screwing around to get it there.
WINDOWS 2.x (1987) Windows 2.01, Windows 2.03, Windows/286, Windows/386 PC, XT, or 386 Class IBM PC with at least 512K of Memory, 1MB + Reccommended, 6MB of Disk Space, EGA/VGA suggested Still can't think of an imporant or influential application. I do know there were some early Voyetra Sequencer releases for Windowos 2.x, as well as I had a weird old Fractal Trees program for it a long long time ago. Windows 2.x never really gained much attention in it's original time either, as people were still not sure what benefit a GUI would bring to their MS-DOS controlled PC. This was around the etime the Apple/PC rivalry really started to get off the ground, with Apple users saying "I don't have to f*** around so much or learn a bunch of gibberish to use my computer" while PC users were all saying "I'm a REAL computer user because I can actually use those COMMANDS to run my machine and actually learn them, and my computer is designed for WORK, not playing with music and dpretty pictures like your Trendy Apple toy!". Of course, we can all laugh at the PC as well for that atrocious and super-cringey Windows 386 rap commercial they made with the woman dressing like a Spy as she obsconds with some files from her Compaq Deskpro 386.
WINDOWS 3.0 (1989) Windows 3.0 The ideal for this was a 386+ macine with 2MB or more of RAM (4MB reccommended), 8MB of Disk Space, VGA or SVGA graphics (OEM drivers for SVGA required). This one was a little influential on it's own in a way, as this was the first version to bring us PROGMAN.EXE - aka, the wIndows Program Manager interface, with it's 3-D Buttons, 16-color icons, Z-Soft Paintbrush as mspaint.exe standard. It also had the famous Chess wallpaper, was the last version of WIndows with Reversi as a game, and I'm sure many people used the Terminal application to dial up their local BBS. Windows 3.0 was when Windows started to get a bit more attention by having it preloaded on certain computers at the time. That said, though, it still did not go as far as Windows 3.1x did. Multimedia required a "Windows Sound System" add-on for sound, Networking was still done at the Real-mode DOS level with LAN Manager or LAN Client (or Novell Netware, or whatever else they had). I call this one the Calm Before the storm.
Windows 3.1 (1992) Windows 3.1, Windows For Workgroups 3.1, Windows 3.11, Windows For Workgorups 3.11, Windows 3.2 Chinese Edition, Windows for Pen Computing 386+ machine, 386DX or 386SL being the best, with 4MB or moree RAM, 8MB of disk space, SVGA graphics, and possibly a sound card and Cd-ROM for Multimedia functionality. Network Interface Card needed for using the "For Workgroups" portion of For Workgroups. Windows 3.1 was the first major version of Windows to actually catch on. Even then, it was not THAT widespread. Windows 3.1 typically shipped on a 386 SX/DX/SL or 486 series machine brand new back then, and I'm sure at least a nice chunk of the user-base never realized it was Pre-Loaded until much later if the maker did not put "WIN" at the end of the Autoexec.bat file. Between releases, Windows 3.1x grew from being almost like 3.0 with Multimedia support wrapped in, into being something like a 16-bit Windows with Windows 95 functionality as For Workgroups 3.11 could - to this day - be connected to a broadband intenret connection using the TCPIP32B add-on driver, and a nice ethernet card. This is also when games for WIndows became more widespread with Debris, Hyperoid, TDK Pinball, Hugos House of Horrors, 221B Baker Street, as well as Sierra making tehir own version of the SCI interpreter to run thheir DOS games in 3.1 with full sound and graphics - such as Kings Quest V or Freddy Pharkas - became commonplace.
Windows 9x starts with the highly influential 95 and ends with the DOS-Burying Millennium Edition. These versions were still based off of DOS (for the most part) and most of the regular DOS rules apply, except the UI and DOS were far more entwined with each other, almost to the point of DOS acting like a bootloader for 9x, but acting as it's own O?S in "MS-DOS Mode". It signifies the shift to a Windows dominant PC vs. a DOS dominant one. It also is the beginning of bloatware, feature creep, and security concerns with Windows regarding networking and internet.
Windows 95 (1995) Windows 95 (1995), OSR1 (1995), OSR2 (1996), OSR2.5(1997) An intel Pentium 90 or 486 DX4-100 machine or better with 16MB+ of RAM, a 2+GB HDD, and SVGA Video, with a modem, soundblaster compatible sound card, and 2X+CD-ROM drive Office 97', MSIE 3/4/5, Netscape Navigator 3/4/5/6.x, Microsoft Encarta, Microsoft Money, AOL Instant Messenger, WinAMP, DOOM95, Quake, GTA, Red Alert Command and Conquer, Descent, Descent II, The Curse of Monkey Island, Midtown Madness, Microsoft Flight Simulator 5 Windows 95 kicked off in early 1995 with a massive advertising campaign licensing music from the Rolling Stones - "Start me Up" - to sell their new O/S. People immediatley liked the new, cleaner, easier to use interface, and it's internet-centric focus compared to clunky old 3.1. It was the perfect storm at the perfect time: The internet becoming mainstream, making PC's mainstream, driving Windows 95 to become mainstream, and these three things standardizing and forever changing the face of what then was still called the IBM Compatible PC. However, not all was perfect as new technologies such as USB and Plug'n'Play were problematic in these early times, and the high resource requirements for a stable system - what I just listed - frustrated long-term-PC-users like myself to no end, as we now had to go out and fork over $1500+ for a brand new "budget" 486 DX4 system, or more mainstream approved, Pentium system in the 75+MHz bracket which would be upwards of $2000. It also was heavy on HDD space too, Windows 3.1x took up 8MB of space on a typical 250MB-2GB HDD of the day, but Windows 95, it used up an entire 150MB just for a BASE LEVEL installation - eating up over 1/5th of your 528MB HDD on an older 486 system. It was so bad a comedian made a video about it based around "Start It Up" with lines like "It's Windows 95, I need a new hardd drive, It makes a Pentium Fly ....My 3-8-6, ain't got the speed, I gotta get myself a brannnnd new machayinnnnnnnneeee".
Windows 98 98 (1998), 98 SE (2001) Intel Pentium 100 or faster with 16MB of RAM+, 56K Modem, 250MB HDD space (so 4GB+ HDD), Sound Card, SVGA or 3D Accelerated Graphics Office, Works, Money, Encarta, IE 4/5/6, Netscape 4/5/6, Quake, Half-Life, Diablo II, The Sims Windows 98 came out in 1998 and while 95 was criticized for it's high resource usage it was beginning to become obvious the "Bloat" was here to stay. Now 98 came with MSIE 4/5 BUILT IN as a part of the Windows explorer graphical shell, eventually prompting a Anti-Trust Lawsuit in both America and Europe over anti-competitive practices in the software industry. In America this was settled more peacefully, but In Europe it lead to IE being removed. Windows 98 SE is the best version of Windows 9x you can get, as it was released AFTER Millennium Edition (Me) to address concerns about Windows Me.
Windows Millennium Edition (Windows "Me") Me (1999) INtel Pentium II, 32MB RAM, 8GB+ HDD (uses up almost or more than a Gigabyte of Space in large installs), Sound, 3D Accel Office, Works, Browsders, know, the usual for the late 1990's Windows Millennium Edition, supposedly supposed to be pronounced "Me" not "M-e" like a lot of people do, was the version of Windows Microsoft was claiming "DOS is Dead" over. Truth was, this was the same as the rest of the 9x series with DOS still included with it, but all sorts of nefarious cover-ups done to hide the fact there was still DOS hiding underneath the beast that was Windows Me. When it was released, it was notoriously unstable, buggy, and had a lot of problems, earning it the same horrid reputation put with Windows Vista almost 10 years later. However, in recent years, it's gained a bit of a reputation of being a pretty good O/S for late 90's PC builds since hte internet is no longer targweteing it. A common fact most people gloss over or forget was this was the "home edition" counterpart of "Windows 2000 Professional" - aka NT 5.1.
Windows NT 3.1 Designed in the image of Windows 3.11 for Workgroups, NT 3.51 was the first NT-based Windows. Windows NT, at this stage, was primarily Enterprise focused and thusly not very widespread. In my whole time of collecting vintage computers, I have yet to have seen a single device with this installed on it.
Windows NT 3.51 Windows NT 3.51 was redesigned to work as an enterprise version of Windows 95 - as obvious by the move from a 32-bit PROGMAN.EXE to the Explorer.exe interface, which I believe YouTuber David Plummer had a hand in converting it over to. Again, it did not see a lot of use outside the office and thusly is not exactly a good candidate for any gaming-centric retro-PC builds.
Windows NT 4.0 Windows NT 4.0 was like the 98 SE of the NT line. IT was held in high esteem and designed to be more like Windows 98, but was far more stable and reliable. However, again, it still had not caught some foothold in the home market at all, so nobody really saw it outside of work, but it was well known as being a good Enterprise O/S and is slikely a big part of why we still use Microsoft Windows in corporate environments until this day.
Windows 2000 Professional Windows 2000 Professional was the NT Equivalent of Me, but unlike ME, it was an excellent O/S off the bat, and shared two traits with Windows 3.1x in that it helped popularize the NT Kernel based Windows as some people bought this for at home - just like Windows 3.1, and also just like Windows 3.1, it enjoyed a very long shelf life because it was solid, reliable, a bit more secure than previosu versions, and was just a good, basic, reliable workhorse. If you're looking for a good NT Version, I'd suggest 2000, specially if you are running something as old as a 486, as it can be tuned to run some XP class software.
Windows XP Windows XP was the last version of Windows to be dominantly 32-bit (x86), and was also one of the longest used and most enduring, with a shelf life of 12 years, with support ending in 2013. Windows XP was solid, reliable, fast, it had a few Security scandals ealry on and some serious teething issues, and the designer interface is bloody ugly ("Windows Fisher Price" we used to call it), but XP is one of the higher up levels of vintage Microsoft Windows more ideal ffor a circa 2001-2004 gaming machine. It was also the first version of Windows to be 100% on NT in it's day, with special editions being HOME, PRO, ENTERPRISE, and ULTIMATE, all in the x86 (common) and x64 (rarely used) versions, as we had not quite made the jump to 64-bit yet.
Windows Vista Windows Vista had a bit of a scandalous and interesting beginning. ARound 2005, Microsoft claimed they were ditching the entire original Windows NT codebase and working on a new project called "longhorn". Windows Longhorn was what eventually was scrapped and became Vista. Due to it's isses with resource usage, security holes, and so on, despite being availible for free to "hackers" with an offer of $10,000 for anyone who could hack the O/S.
Windows 7 Another long-run Windows, like 3.1, 98 SE, 2K, and XP, was Windows 7. Windows 7 was when the x86-x64 crossover began to take hold, with more 64-bit installations coming later, and support for (U)EFI being rolled into the O/S, allowing for less partition waste using GPT partitions among other things. It also was the beginning of widepread use of Bitlocker drive encryption in enterprise environments, whereas previously you had to have a 3rd party product like Sophos to do that for you. Windows 7 officially reached end of life in 2021, with paid extended support from Microsoft continuing up to 2024 for Enterprise users and rich people. 7 is a great O/S to use for a tweener because it actually allows you to plug later era ATA HDD in via USB and write DOS and Win9x installs to them via drag'n'drop, which I STILL use Windows 7 for.
Windows 8 Windows 8.0 (2013), Windows 8.1(2014) In 2013, the PC Industry was in trouble, sales of PCS were down, and sales of touch-screen mobile devices were up. Not sure if Microsoft remembered Windows for Pen Computing circa 1992, or if they forgot, but I do know it wwas almost like a coming-back-around of the attempt to get people into Pen/Touch/Tablet PCs at the time. The interface was redesigned to be consistant with the failed "Windows Phone" product - a WIndows-based Smartphone - eliminating the taskbar and start button at first, which the button and bar were returned in Windows 8.1 by user request, but the Start Menu was still replaced with the "Metro" or "New Style UI" that Windows used that had it's own special applications in special tiles designed to work and be used like a touch screen phone application. Microsoft Edge was a part of this rollout, though under an entiretly different codebase at the time.
Windows 10 Windows 10 was released in 2015 to return to the standard Windows Form, though the "touch/pen" features were still available as a user selected option. Windows 10 was an interesting release over time, as it was more like a "rollercoaster" than an easily pinnable "dung heap" or "massive success" on Microsoft's part. Part of this was because it had rolling "Builds/Revisions" based on numbers such as 1803, 1909, 20H2, 20H1, instead of Service Packs or Official Service Releases like NT/2K/XP/7 or 95 respectively. Windows 10 is still current but it's when we start to lose more and more legacy features such as support for FAT-16 formatted drives, and the ability to read hard disk drives with DDOs on them. It also was the era when the changeover from the old BIOS Int13h boot scheme used since the original IBM PC in 1981 was replaced by the Universal Extendable Firmware Interface (UEFI) used these days, with a GPT (General Partition Table) hard drive scheme that wastes less space with management data, allowing for more usable space from the unformatted space on the drive.
Windows 11 Windows 11 was released in early 2022, and is the latest version of Windows by Microsoft. The major changes once again included an attempt to move to a new style of UI, this time mimicing the Dock Bar from Apple, which was lifted from a *nix distribution stylistically.