SCSI is an acronym standing for Small Computer System Interface. Originally describing the interface between a computer and external peripherals, such as printers, scanners, external drives, etc., it is now also used to describe the interface between the motherboard and certain internal hardware (typically drives).
Along similar lines, people tend to associate IDE with drives that use the 40/80 pin parallel cables. This was also known as ATA (AT Attachment). With the onset of serial drives, the older standard was renamed as PATA (Parallel ATA) and the newer one as SATA (Serial ATA). In all cases, the drives are still IDE (Integrated Drive Electronics) devices. All this means is that the controller circuitry for the device itself is mounted on the device, with the advantage that the controller can be designed for the device instead of having to be generic (as was the case when the controller was on the motherboard, either natively or via an add-on card).
As a footnote, my drives are described as SCSI drives, even though they are SATA units.