ATA33, ATA66, ATA100 and ATA133, each number corresponding to the transfer rate in MB the bigger the faster. This type is attractive standard. If your computer is older, you may only be able to use this kind, and depending how old will determine which speed.

SATA is a latest kind. It uses a different cable and allows much faster information transfer. The slower kind is 150MB and the faster is 300MB, as a result they are much faster than IDE.

A latest technology called Native Command Queuing speeds up how fast things are found and done on the hard drive. Just temporarily, it orders the read and writes commands given to it in like way as to get the tasks done earlier and with less delay.
Drives are most often separated by their size and this is a big factor in the pricing of a drive. The minimum drives today are around 40GB, you can get a little smaller, and the largest of 400GB will be enough for a really long time.

The other thing that affects pricing is the rotation speed, which is the speed at which the big disk spins inside the disk drive. The faster it spins the faster information gets found. Look for rotation speeds of 7200RPM as standard and 10000 for the fast end.

For a major drive, which has your Windows and programs on it, go for something faster, but bear in mind that even for me my ATA66 drive is fast sufficient for my uses. But a faster one will speed up how fast your programs go