The three most common types are E(IDE), SATA and SCSI
Forget about SCSI, they are DAMN expensive.
IDE was used until a few years ago, then EIDE came...the one u are probably using in your computer. EIDE is enhanced IDE, so basically an improved technology.
SATA is the newest technology.
All motherboards support (E)IDE and most newer motherboards support SATA (wich is faster than (E)IDE)
Make sure ur motherbpard supports SATA before buying a SATA drive
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....