MIDI is short for Musical Instrument Digital Interface. This standardized format has many benefits, but their tiny file size is high on the list. Instead of recording audio tracks, which take up a significant amount of space, MIDI files use existing audio tracks and arrange them. Because the MIDI files don’t have any actual audio, they are slightly less portable.
I’ve written several articles on MIDI – what it is, what it can do, etc. You can see all of my articles that have anything to do with MIDI at the MIDI tag page here: http://www.homebrewaudio.com/tag/midi/
For a different take on it, check out the post at AudioTuts below.
You can read more here: http://audio.tutsplus.com/articles/general/whats-this-midi-all-about-then/