The difference between a disk and a disc is pretty simple, actually. A disk refers to a magnetic storage device that is not circular, so we have a floppy disk and a hard disk (Yeah, it's just the casing that isn't circular, I know, I know). Conversely, a circular storage device is a disc. So a record is also a disc, and we have compact discs and digital versatile discs. This is also why you listen to disc jockeys and not disk jockeys.

The fourth edition of the American Heritage Dictionary of the English Language (available via agrees.