Well, there are several parts of a hard drive that may confuse some, but luckily we have "Backwards Compatibility" with standards so it's much easier to not make a huge mistake...
Your hard drive will likely be either a SATA or IDE/ATA drive. Now each of these specifications have been updated in recent years for speed (After all, we don't want to wait 5 minutes to access a file do we?).
SATA now has 3 Gbit/s (with draft specifications for 3Gbit/s), and ATA is ATA/133 (that'd be 133 MB/s); however you can use an SATA3g drive in a simple SATA connector, as you can do the same with a ATA133 disk in an ATA adapter; which just means you can put a faster drive in a slower slot; which you'll have to end up doing as it'll be harder and harder (and even more expensive) to find a older hard disk for outdated standards (and I can't understand why you'd really want to).
If you take the short time to glace at the wikipedia links above, you'll see that the SATA standard is far faster than ATA; it's why it's generally globally recommended.
