MIDI is instructions, not sound
A MIDI file contains no audio. It records performance events — note on, note off, which pitch, how hard, which controller moved — and the sound is produced by whatever instrument interprets those events. The same file played through two different synthesisers produces two entirely different results.
This is why MIDI files are tiny, often a few kilobytes for a full arrangement, and why they were ubiquitous in early web and game audio. It is also why a MIDI file that sounded good on the machine it was made on can sound wrong elsewhere: General MIDI standardised a set of 128 instrument numbers so that program 1 is a piano everywhere, but the actual sound of that piano depends on the playback device's sound bank.
Note velocity is expressive data rather than volume alone. A well-recorded MIDI performance carries the dynamics of the playing, and most instruments change timbre as well as loudness with velocity — which is why flattening all velocities to a single value makes a performance sound mechanical even at correct timing.
Timing, ticks and quantisation
MIDI time is measured in ticks, and a file's header declares how many ticks per quarter note — commonly 96, 480 or 960. Higher resolution allows finer expressive timing. Tempo is stored as a separate meta event in microseconds per quarter note, so a file can change tempo throughout, and the tick grid stretches with it.
Quantisation snaps note timings to that grid. Full quantisation produces perfect machine timing and usually removes the life from a performance, since the small deviations from the grid are much of what makes playing sound human. Partial quantisation — moving notes a percentage of the way toward the grid — tightens timing while preserving feel, and is almost always the better choice.
Swing is a deliberate departure from even division, delaying every second subdivision. It is fundamental to jazz and much popular music, and quantising a swung performance to a straight grid destroys it entirely — one of the more common ways to ruin a recording while apparently improving it.
Channels, formats and what transfers
MIDI defines 16 channels, and channel 10 is reserved for percussion under General MIDI, where note numbers select drums rather than pitches. A file whose drums play as melodic notes has usually had its channel assignments changed.
Files come in three formats. Format 0 merges everything into a single track, which is maximally compatible and awkward to edit. Format 1 keeps separate tracks sharing a timeline, which is what most software produces and what you want for editing. Format 2 holds independent sequences and is rarely encountered.
The practical limitation to remember is that MIDI captures notes and controllers, not sound design. Exporting a track as MIDI transfers the performance but not the instrument, the effects or the mix — so a MIDI export of an elaborate synthesiser part arrives as a bare note sequence. For that reason MIDI is excellent for exchanging musical ideas, arrangements and notation, and inadequate as a delivery format for finished music.