Most home and small-office PCs use an IDE hard drive and have a
PCI bus for adding components to the computer. But a lot of computers, particularly high-end workstations and older Apple Macintoshes, use the Small Computer System Interface (SCSI) bus to connect components, which may include:
SCSI devices usually connect to a controller card like this one.
Basically, SCSI (pronounced "scuzzy") is a fast communications bus that allows you to connect multiple devices to your computer. In this edition of HowStuffWorks, you'll learn about the structure of SCSI and the various specifications and types, as well as SCSI IDs and termination.