What is MIDI? MIDI music or do you mean MDI (multi document interface)
a good introduction is at
http://hotwired.lycos.com/webmonkey/97/21/index4a.html?tw=multimedia
see also
http://www.soniccontrol.com/tech/midi/faq/
from that site:
What is MIDI?
MIDI stands for Musical Instrument Digital Interface. MIDI is a
standard that lets your MIDI device (like your MIDI keyboard)
communicate with other MIDI devices and your computer. When you hit a
key on your keyboard, a series of controller messages (sets of digital
information) are sent to your computer and interpreted by your
sequencer. These controller messages contain a wide range of MIDI
information that translate the aspects of your performance into
electronic MIDI form. An example of a MIDI control message would be
the Note On/Off message. When you hit a key, you are triggering a midi
event, or turning it "on". When you release the key, the midi message
is stopped, thus turning it "OFF".