a CD is that cool little round thing that you put in a slot
a Hardrive, the just of it is.. its basicaly a round metal disk that spins reeeallly fast and stores magnetic information
@nna2 You sir, are being quite rude. I think it would of been better for everyone if you didn't make that post. Just putting in "my 2 cents".
@Albino Hard drives are also referred to as HDD - they are the most common and economical internal storage solution today. They differ from discs [i.e CD/DVD/Blu-ray] as they are a form of optical media and requires the right hardware to be played. All discs can be physically mounted and removed (assuming you have the right hardware, i.e CD rom, DVD drive, blu-ray drive, etc).