Where MIDI files are concerned, this is like the paper reel in a player piano; it is a file full of notes which can then be played back by the soundcard of your computer or on a keyboard or (less likely) another instrument that supports MIDI files.

Note that MIDI files are not recordings, just a series of notes.  It is analogous to reading these words on a page.  They can get read aloud, but do not have a voice associated with them.  This may seem odd, but there are a number of things you can do with a MIDI editor (I still use the out-of-production MIDI Orchestrator Plus from Voyetra).

• Multi-tracking.  MIDI files are usually multi-tracked.  Each instrument is on a different channel (up to 16 different ones).  These may be independently edited.

• Voices.  The notes play in any number of sounds.  This means you can have the guitar part played by trumpets or the string section replaced by a pipe organ.

• Editing.  You can also delete tracks or or add entirely new tracks.  Many editors allow you to even paste in tracks from other files.  Further, you can change individual notes, both in pitch or value.  You can also add or delete individual notes to modify chords, etc.

• Tempo.  The tempo may be altered, even in the middle of the piece.

• File size.  MIDI files are very small compared to a digital recording of comparable length.  Also, they can be compressed to a tiny fraction of their size (smaller than 25% of the original file on average), so you can fill your hard drive with quite a library without taking up too much space.

• Scoring.  Finally, by having a multi-tracked file containing all the notes to a particular song, you basically have the sheet music score to it.  Often, I have looked for files just to learn to play a guitar part.

• Play-back.  Because MIDI is triggering a "recording" or "synthesized" sound, the same note is essentially the same every time (unless there are dynamic "filters" like flanging being applied, but these only work to some extent).  The human ear notices this lack of variation and finds it cold and sterile.  Similarly, MIDI sequences are often "robotic" in that they hit every note perfectly.  Humans cannot do this, but it sounds more natural when things are a little "flawed."

• Vocals?  Sorry, no vocals.  Since MIDI isn't an audio recording (i.e., all the "audio" resides in your synth and/or soundcard; the midi file just calls it up), you can't record vocals in the file itself.  This is one of the frustrating things about MIDI files.  They're all instrumental, although the melody line is often present in the files found on the web, but these can sometimes be reminiscent of grocery store Musak.

• Limited palates.  No matter how good a synthesizer, it can never replicate all the possible tones, so a MIDI reproduction will always be different than the original.  This is especially true when listening to MIDI files on a soundcard.  Even there, some cards will contain better voces than others, so some files may be fine on some machines, while on others they may be almost unlistenable.  Unfortunately, when you play a MIDI file midi on your computer, you are stuck with just the 128 sounds on your soundcard.  If you try to reduce the entire range of instruments to that small a number, you're sure to make a few sacrifices.  Musicians who create sequences do their best to approximate the sound of the original instrument in the MIDI file based on what it sounds like on *their* machine, but you never know what it will sound line on someone else's.