MIDI is an abbreviation for Musical Instrument Digital Interface. Instruments that support MIDI can communicate and control each other. The most common MIDI instruments are electronic keyboards, but there are MIDI versions of wind instruments and probably most others as well. Ordinary PCs can usually be used with MIDI keyboards via a cable plugged into the joy-stick port.
A MIDI file is not a recording of musical sounds, but of one or more human or computer generated musicians' performances. It is easy to completely change such a recording - for example by changing the instruments it is played on. MIDI recording and editing is vital to the production of most pop music today. If you have ever been to a karaoke club or night then you may have used MIDI files without realising. Karaoke machines use an extended form of a MIDI file to play music. That is why it is so easy to change key or speed a song up or slow it down when using them. For a link to one of the sites that taught me a lot about MIDI click here. PG
When it comes to the Internet, MIDI files have one great advantage over the most popular form of Internet music (MP3): they are far smaller. The two minute extract of Harpo is ten times the size of the midi file for the whole piece of music (which is over eight minutes long). At CD quality, it would be sixty times the size. Not only that but MIDI files can be compressed by programs like WINZIP. There's no hope of compressing an MP3 file like this. However MP3 files will sound the same anywhere and contain can vocals - MIDI files can only contain lyrics.
There is a hybrid form of music called MOD (module) or tracker music, where files have information similar to MIDI files, but with sampled sounds as well. This makes the files a lot bigger than MIDI files but usually much smaller than MP3. There are many different file formats. There used to a big MOD scene on the Internet, but I am not sure if there still is - all the links that were on the old version of my web site are now broken.