SCSI or "scuzzy" as it's often pronounced, is just another interface used by drives. It is most common in servers, as many of them can be linked together with one cable, where as with IDE, you can only have 2 drives per cable, and SATA with 1 drive per cable. SCSI also supports higher transfer rates that IDE, I believe, which is why you see 15K RPM SCSI drives and only 10K IDE drives. If you are building a computer, unless you are a serious gamer or wanting to build a server, SATA is the way to go.