SCSI is a set of standards for physically connecting and transferring data between computers and peripheral devices, such as a hard drive or CD or DVD drive. SCSI stands for Small Computer System Interface, although the "small" is historic; SCSI is used on both large and small computers. SCSI is usually pronounced as "scuzzy".
Few peripherals (other than hard drives and tape drives) were made after the 20th century. Later ones used USB or in some cases Firewire.
SCSI is realized by a bus slot on the motherboard. Just to put a card called host adapter to the motherboard. From this card is a managed data cable. This cable connects individual SCSI peripherals. Each of this peripherals has its own sequencer which is functional independent. Operation on data cable (bus SCSI) is controlled by the host adapter. This makes it possible to install
external components. Transferring data on the SCSI bus is managed by the host adapter. The CPU is not loaded by transferring data.