MIDI (an acronym for Musical Instrument Digital Interface) is a technical standard that describes a communications protocol, digital interface, and electrical connectors that connect a wide variety of electronic musical instruments, computers, and related audio devices for playing, editing and recording music. The specification originates in a paper published by Dave Smith and Chet Wood then of Sequential Circuits at the October 1981 Audio Engineering Society conference in New York City then titled Universal Synthesizer Interface.
A single MIDI link through a MIDI cable can carry up to sixteen channels of information, each of which can be routed to a separate device or instrument. This could be sixteen different digital instruments, for example. MIDI carries event messages; data that specify the instructions for music, including a note's notation, pitch, velocity (which is heard typically as loudness or softness of volume); vibrato; panning to the right or left of stereo; and clock signals (which set tempo). When a musician plays a MIDI instrument, all of the key presses, button presses, knob turns and slider changes are converted into MIDI data. One common MIDI application is to play a MIDI keyboard or other controller and use it to trigger a digital sound module (which contains synthesized musical sounds) to generate sounds, which the audience hears produced by a keyboard amplifier. MIDI data can be transferred via MIDI or USB cable, or recorded to a sequencer or digital audio workstation to be edited or played back..
More info on MIDI can be found on Wikipedia - MIDI