The internet, as a whole, is just a giant network. That's it, nothing fancy. And in that giant network, is a series of various "programs" or "systems" that all were created initially to serve one purpose, but their purpose gets diluted more and more over the decades by money hungry opportunist types. Basically, imagine the internet as one big circle that is the whole world, and then picture every "technology" used within it as just one part of that "ecosystem" of sorts.

As we close down from the macro to the micro, or the big things people take for granted, down to the tiny things we use every day, in the middle is the general technologies that all this stuff runs on. However, there is one basic tenant under which ALL of the internet falls: File storage, and File Sharing. You put something on an internet connected "Server", and you're effectively sharing files with the rest of the world.