August is the first month I have to do this. I'm going to cheat a bit, and borrow one that was already run on The Unofficial Apple Weblog (TUAW)
Discs and Disks. Did you know that there is a difference between these two similarly pronounced words? There is. Read below to learn more.
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.
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").
And now you know.