scsi was a competitor interface to IDE/ATAPI interface, and it's a serial interface, similar to SATA, but rather than focus the bandwidth toward a single drive, creating as fast of a connection as possible, it allowed you to link dozens of drives together off a single controller.
it was primarily used for servers, and the first RAID arrays were SCSI.
Now there's a new version of scsi called SAS, which is supposed to compete with SATA, which allows up to ~250 15,000rpm drives to be chained together. So although no single drive has as fast a data transfer rate as SATA, you can link TONS of drives together, to be accessing HUNDREDS of different files/segments at one time. And 15,000rpm drives have the fastest seek times of any hard drive to date.