Many manufacturers of electronic musical instruments and sound cards use the General MIDI program change list to assign program numbers to their sounds. This is done to create uniformity in the selection of sounds. Regardless of the sound card you're using, you can be reasonably confident that you'll hear a piano sound, for example, if you assign Program 1 to a track in your score. Additionally, if you save scores as MIDI files to be played by sequencers on other instruments, your program assignments will remain reasonably consistent regardless of the instruments on which the MIDI files are played. This assumes, of course, that the manufacturers of those instruments adhere to the General MIDI program change list. Most newer instruments will conform to this list.
General MIDI Program Change Numbers lists the sounds associated with MIDI program change numbers 1 to 128.