there's 2 types of CD Images for the PC that are considered mainstream. there's .iso, which used to be big, but recently it's become .bin/.cue files. .iso files hold the entire cd image and is pretty much a big zip file. .bin files work the same way, except they require a .cue file (they ALWAYS come together) which basically tells the burner how to burn the .bin file.
.bin files have a big plus because you can burn games that have audio tracks on them as well, seperately. for example you can burn the data, and seperate .wav (or music files, a soundtrack) to the same CD using the .cue file. for example the Quake 3 CD has a seperate audio track you can stick in a regular CD Player to listen to i believe.