Hard drives are available in two basic flavors: SATA (serial advanced technology attachment) and IDE (integrated drive electronics). IDE drives use a 40-pin connection (look for the wide, flat cable). They are the more common drive format and tend to be cheaper. In the past few years, drives using a faster, simpler SATA connection have come to market. SATA drives tend toward higher capacities and transfer data at six times the speed of IDE drives.
Remember that data transfer is only as fast as the slowest connection, so the higher speed of a SATA drive is wasted if the data is handed off to a USB 2.0 cable that moves bits at one-sixth the speed.