I like Escamillo's explanation of Midi. Spot on. To put it into an even clearer perspective, think of the piano roll used on old player pianos. The perforations and imprints on that piano roll, (while providing no musicial sounds themselves) instruct the piano mechanism on which notes and in what combination and timing to depress. Midi does the same thing to the sound source it's connected to. Obviously the more voices your sound card is capable of representing the more accurate those sounds will be. If it is an entry level or very basic sound card, the sounds triggered will only provide an approximation of how the song was intended to sound.