SMC (Super MagiCom) is the most common format used for Super Nintendo Entertainment System game ROM images. SNES emulators such as Snes9X and Zsnes understand this format and can play the games. The main distinction between SMC and FIG (the other common SNES ROM format) is that SMC files have headers added to the game data that describe the game and how the SNES accesses it, while FIG files do not.
.smc is also the filename extension generally used for headered SNES ROMs, often regardless of whether they are in SMC format or not. There are a number of ROM formats with headers and they are all identical to anyone who is not an emulator programmer; these include the very common Super WildCard (SWC) format. Nach's SNES ROM Tools recognise the ROM format and can change the extension to match the format. However, this is not necessary to play ROMs in an emulator.
Since the .smc extension is so universally applied to SNES ROMs, it may be a useful key for finding them on file sharing networks such as Gnutella.
For more technical details and information on converting ROM formats, see http://pt.parodius.com/help.html . | 
