The term "MIDI" stands for Musical Instrument Digital Interface. It was originally conceived as a way of connecting electronic musical instruments together so they can "talk" to each other -- so you could play one instrument from another.
MIDI data is neither music nor sound, it is only "control" information. It provides a way of having a "controller", such as an electronic keyboard, send out data describing which notes were played, when, how hard they were hit, how long they were held, if pressure ("after touch") was applied after a note was struck, if pitch bend is being employed, if a patch change (sound change) is being called for, and so on. Another way of thinking of it would be if you could take the keyboard off of a piano and walk across the room with it, leaving the rest of the piano behind. All the "stuff" you would need to stretch between the keyboard and the rest of the piano (so you could still play it) would be what gets packed into the data in a MIDI file or into the electronic signals flowing through a MIDI cable. In a MIDI setup, this control data is fed from a MIDI file or a MIDI keyboard into a slave MIDI sound generator such as a synthesizer, sampled audio playback unit, or a PC sound card (in a sound card, the sounds can be synthesized or can sometimes be sampled real sounds which are sometimes called "wave tables"). The MIDI sound generator responds to these MIDI commands and generates music, just as an orchestra would in response to the command of a conductor or a player piano would in response to a piano scroll. If the controlling MIDI keyboard and the MIDI sound generator are all in one unit (not separated), the combination is often called a synthesizer. If the MIDI keyboard makes no sound on its own and only sends out MIDI data, it is called a "controller keyboard." If a MIDI sound generation device has no musical keyboard but only makes sounds in response to incoming MIDI data, it is called a MIDI sound module.
Recording MIDI data onto a computer file as someone plays at a MIDI controller keyboard (or other controlling device such as drum trigger pads) is called "sequencing." Sequences stored on computer can be edited to improve the recorded musical performance. Sequences can also be overlaid or overdubbed to produce several parallel MIDI "tracks" which can be played back simultaneously with different instruments sounding on one or more MIDI modules, simulating an entire musical ensemble. Today's MIDI modules can play back many different sounds at the same time (piano, drums, bass, strings, and so on), which is called being "multi-timbral." That way all one needs is a MIDI controller keyboard, a computer with MIDI sequencing software (and a MIDI hardware adapter), and one or more MIDI modules or a good PC sound card -- and you have a band!
