Typically, these files are created by modifying a "Piano Roll" of sorts - which is basically a "grid-sheet" like interface where notes are represented in "columns" and their space in the time signature is listed in "rows" - almost like a "musical spreadsheet".

These rows are cut up into intervals of usually 16 - for 16th Notes, though they could be cut up as much as 64 or as little a 4 spaces for notes in a really crappy editor nobody's ever heard of. Sometimes it's adjustable, sometimes not. For example, BandLab cuts it's piano roll up into 16-note measures.

The Rows usually range from C0 all the way to C8, allowing for a wide range of notes. In earlier MIDI, this meant some pretty strange, out-of-character notes were possible on all instruments, but more often than not modern sequencers LIMIT these notes within the natural range of the instrument being played.

But a second use has also been found in using MIDI as a differnet kind of controller. Often professional musicians will use MIDI, even us guitarists, in our rigs to controll various footswitches or banks of effects all at once. Bill Bradshaw built a CAREER around building such units for such famous musicians as Eddie Van-Halen and Steve Vai. The advantage this gives us is the ability to control an Amplifier head and/or a bunch of Analog and Digital Effects, all through one interface on stage, and retain the sound quality of which we want to retain from the studio while playing live. In some cases, these effects can be tied to a digital backing section such as sequenced keyboards - popular on 80's pop and rock tunes - to help out the keyboarrdist, and then the effects changes can be "automatically" put into the set over MIDI, allowing the guitarist or bassist to focus on playing the song, rather htan stomping a bunch of footswitches to change the settings on stage. That migth explain why say, you're watching The Cars on their Heartbeat City Tour in 85', and Elliot Easton is at one side of the stage, his pedals on the other, than somehow he's got a chorus switched on - because it's likely tied to Greg Hawkes synthesizer stack - which he NEEDS MiDi to control multiple layers of synth to do some of those highly complex arrangements live. Maybe not, but that's another way professionals can employ MiDi in a LIVE context.

And to use Greg Hawkes as another example - say Greg Has four keyboards he needs to make one sound (especially rlevant to the 1980's when each analog and digital synth had a "personality" to them), and needs them ALL synched at once - he can't just spawn 2 more arms to make that complex sound - so he's got them all networked over MIDI, and they are all talking on the same channel for that sound - so Greg can play 4 keyboards at one time, on one actual keyboard. Say he has a modified minikorg 700S with a MIDI controller attached, a ARP Solace, a Sequential Circuits Prophet 5, and a CMI Fairlight - just for giggles - and all of these have, or were modified to have MIDI Controllers installed - so Greg can hit his Prophet's keys and trigger any or allof them at the same time for complex lines. Again, not faking it - this is controlling a large amount of sonic firepower from ONE keyboard. You have one keyboard with a pad that fades in, the Touch Sync on the Prophet (think "Let's Go"), Square Lead on the Korg, and the ARP doing an older 70's string synth sound.