Hi Marie,
MIDI stands for
[Musical Instrument digital Interface] which is a common conputer language.
It is the modern equivalent to the player piano type of music where a small computer file controls (in this instance) your sound card. AS an example, it may tell the sound card on your computer to start a flute sound that is of a certain volume and then later it may tell the sound to stop. The file for a given MIDI piece is relatively small compared to a live recording. One advantage in using MIDI files is that you can (with the proper software) control the speed of the playback and increase your technique as if using a metronome. You can hear the pitches of the piece and the accompaniment to improve timing and tuning. The downfalls are that the sound generally isn't all that sophisticated (Automated Machine) unless the programmer uses a more complex parameters and/or you have a really good sound card that uses advanced sampling to produce the sounds. A good recording can sound pretty realistic and it is sometimes used in actual performances. TV backgrounds and local musicals are places where you can often hear it. For me, it seems that my instrument the flute never sounds realistic. For others their instrument sounds more fake.
Duets can be bought at local music stores or from fluteworld.com. or:
http://fluteworld.com/index.php?action=pk&wart=04a
at that place, they have a grading system of 1-5 for their music so you can choose your level. In my teaching, I use the Rubank Selected Duets to advance rhythm and independence but other more modern duets can be good fun.
~Bilbo
N.E. Ohio