SNES SPC files are raw memory dumps of the snes sounds chip when a game sends sound info to it. Most PC SNES emulators have a dumping option.
MegaDrive GYM files are a logged sound format and is the sound info dumped over time from a megadrive emulator.
NES NSF are NES ROM files edited so that the only play music.
GB/GBC GBS files are rom files edited down until only the sound plays
PSF, USF, GSF etc are all the ROM edited down until it only plays the music.
MOD is kind of a midi/wav hybrid, it has instructions on how to play the music like midi, but it also includes its own samples(normally wav) to make the notes and effects.
So there you go a very quick rundown of emulated music formats.
EDIT: forgot to say... the SPC format has a constant filesize and can loop indefinitely, the GYM format depends how much of the song was dumped, the xSF formats depend on how well it was ripped from the rom (can loop indefinitely), and mod files are generally quite small too, depending on the size of the samples included.