From Sega Retro

BIN (binary) files consist of unspecified data and code. In the case of games dumped from cartridges or CDs, it is an exact copy of the data on its original medium, i.e. not compressed or rearranged. The *.bin file extension is commonly used for CD images (such as those of Sega Mega-CD or Saturn games) and is accompanied by a *.cue file which describes where on the disc the data was found. Sega Mega Drive ROMs are often found with a *.bin file extension, or occasionally renamed with a *.gen (and in some cases, as with no-intro sets, *.md) extension. Likewise with Sega 32X and *32x, Master System and *.sms, Game Gear and *.gg, etc. The SMD extension is used for Mega Drive ROMs dumped by a Super Magic Drive and is distinctly not the same as BIN since the file's contents have been rearranged.