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.

These days, your hard drive will likely be an SATA hard drive as IDE/ATA drives have been phased out, but can still be located on eBay.

The speed of your drive will vary based on the specifications that it supports, an SATA hard drive for example:
SATA revision 1.0 (1.5 Gbit/s, 150 MB/s, Serial ATA-150)
SATA revision 2.0 (3 Gbit/s, 300 MB/s, Serial ATA-300)
SATA revision 3.0 (6 Gbit/s, 600 MB/s, Serial ATA-600)
SATA revision 3.2 (16 Gbit/s, 1969 MB/s)

An IDE/ATA hard drive, before being phased out, supported:
Original: 8.3 MB/s maximum originally
Later: 33, 66, 100 and 133 MB/s

It is important to note that, while backwards compatible, the SATA wires in your system could need upgrading when going with a faster drive (assuming your motherboard, or addon card, supports the higher standard).