This post is brought to you by the letters 'c' and 'k'

No, I don't mean "cee dot kay dot", as in C.K. I mean the 'c' in disc versus the 'k' in disk. Apparently some of you are very confused by what the difference is and you aren't sure which one you are supposed to use and when.

Apple's Knowledge Base to the rescue! In article 30152 we learn that they're pronounced the same, but, technically speaking, there is a distinct difference between a disc and a disk.

Discs

A disc refers to optical media, such as an audio CD, CD-ROM, DVD-ROM, DVD-RAM, or DVD-Video disc. Some discs are read-only (ROM), others allow you to burn content (write files) to the disc once (such as a CD-R or DVD-R, unless you do a multisession burn), and some can be erased and rewritten over many times (such as CD-RW, DVD-RW, and DVD-RAM discs).

All discs are removable, meaning when you unmount or eject the disc from your desktop or Finder, it physically comes out of your computer.
Disks

A disk refers to magnetic media, such as a floppy disk or the disk in your computer's hard drive, an external hard drive, and even iPod. Disks are always rewritable unless intentionally locked or write-protected. You can easily partition a disk into several smaller volumes, too.

Although both discs and disks are circular, disks are usually sealed inside a metal or plastic casing (often, a disk and its enclosing mechanism are collectively known as a "hard drive").

I am sooooo glad they cleared that up, aren't you? Remember folks, knowledge is power. Now go impress someone with your newfound knowledge. It's a proven fact that citing from Apple Kbase gives a guy a tremendous edge with the ladies...