What is a disc?
Discs are optical media such as an audio CD, CD-ROM, DVD-ROM (or RAM), or a DVD-video disc. Discs also include recordable media, such as CD-R and DVD-R. All discs are removable, meaning they eject from your Mac.
What is a disk?
Disks are magnetic media, like your built-in hard disk. Disks also refer to external hard drives and are always rewritable, unless otherwise protected with software. Disks can be partitioned into smaller sections and be given different functions.
So, the next time you are having an issue with a DVD that is not playing, you will be able to tell the Genius at the Apple Store that your d-i-s-c is not playing properly. Or, if your internal hard drive is acting up, you can tell AppleCare that you seem to be having a d-i-s-k problem. Obviously, this is primarily useful when writing about your issues, so when you log on to the Apple Support Discussions forums or the CNET Mac Forums to describe your problems to other Mac users, be sure to use the proper terms!