The MIDI protocol defines what data is sent from one musical device to another in order to communicate a musical event. For example when a key is pressed on a MIDI keyboard, a Note On event is sent from the keyboard's MIDI output and then through a MIDI cable and received by another MIDI instrument's MIDI input. This event describes which note was played and how fast (the velocity) it was pressed. The receiving device then interprets this event and plays the appropriate sound.

Links: The MIDI Specification (free), The Official MIDI Specification Documents ($$$)