What is MIDI?
MIDI stands for Musical Instrument Digital Interface. It can be used to refer to the protocol, the interface and the file format. It was developed as a standard so musical instruments could talk to each other and so musical instruments could talk to computers. The protocol sends messages such as note on and note off. Because it is a list of instructions on how to generate sounds, a MIDI file is different from an audio file that actually includes the sounds. Thus, a MIDI file is much smaller than an audio file. A MIDI file is not based on visual notation, but on commands to turn notes on and off.
General MIDI (GM) is a standard list of 128 instruments so files will play back as the composer intended.