This article is from the Electronic and Computer Music FAQ, by Craig Latta Craig.Latta@NetJam.ORG with numerous contributions by others.
(adapted from firstname.lastname@example.org (Joseph D. McMahon)):
MIDI stands for "Musical Instrument Digital Interface". It is an
international standard which allows electronic keyboards, sound modules,
effects boxes, and other equipment to send information to one another.
Possibly the simplest use of the feature is when the MIDI OUT jack of
one synthesizer is connected to the MIDI IN jack of another, allowing you
to play both at the same time using just one keyboard.
MIDI can carry much more than just notes, however; most MIDI-capable
equipment will produce and respond to a variety of messages on the MIDI
bus. Also, the proper interface hardware will allow a computer to
control the musical equipment (see "EDITOR/LIBRARIAN" and "SEQUENCER").
MIDI is often used to build a studio in much the same way that you can
build a stereo system: by choosing individual components and combining
them into a whole.
The standard was instituted in 1982, and most keyboards after the Yamaha
DX7 have it. (Nit-picky note: some built before do, too, but the DX7 is a
good reference point.) It includes both a hardware standard (it's a
5 mA current loop carried on 3 wires, with 5-pin DIN connectors) and a
serial communications protocol based on byte-oriented message packets
running at 31.25 Kbaud.
[for a more complete treatment of MIDI, see the item "Where
can I find out all about MIDI?" -- Craig]