MIDI.com

Clearinghouse of MIDI files, info, discussion, etc:

MIDI is the acronym for Musical Instrument Digital Interface. Throughout its existence, MIDI has gained acceptance with industry professionals by leaps and bounds. Not only does it allow for faster creation and composition of music, it allows the composer to become the proverbial one-man band.
MIDI was introduced in 1983. It was developed in cooperation between the major music industry electronic instrument manufacturers including Roland, Yamaha, Korg and others. No one ever dreamed the kinds of sounds that could be created and accessed using such a powerful communications protocol.

The MIDI protocol allows electronic devices (usually synthesizers, but also computers, light show controllers, VCR’s, multi-track recorders, etc.) to interact and work in synchronization with other MIDI compatible devices. Using a master controller device such as a keyboard, one can play or trigger sounds from other electronic devices remotely. This eliminates the need for one keyboardist to perform with nine or ten keyboards around him. He can play all the keyboards through one simply by connecting them using MIDI. The other keyboards can be off-stage; he never has to touch them, yet he can play them.

The best analogy for MIDI is to liken it to the linking of two computers via modems. The same way the computers share information via modem, electronic devices share it via MIDI. It does not send the actual musical note, but the information about the note. It can send messages to synthesizers telling it to change sounds, master volume, modulation devices, which note was depressed, and even how long to sustain the note.

MIDI has been continually expanded to include other features for the professional musician. Some of note include MIDI Time Code (allowing synchronization of video and audio), Sample Dump Standard (allowing for the transfer of digital audio files) and MIDI Show Control (allowing control of devices used in theater).