Once upon a time, the Small Computer Systems Interface defined both a parallel I/O bus and a data protocol to connect a wide variety of peripherals (disk drives, tape drives, modems, printers, scanners, optical drives, test equipment, and medical devices) to a host computer.
Although the old parallel (fast/wide/ultra) SCSI bus has largely fallen out of use, the SCSI command set is more widely used than ever to communicate with devices over a number of different busses.
The SCSI protocol is a big-endian peer-to-peer packet based protocol. SCSI commands are 6, 10, 12, or 16 bytes long, often followed by an associated data payload.
SCSI commands can be transported over just about any kind of bus, and are the default protocol for storage devices attached to USB, SATA, SAS, Fibre Channel, FireWire, and ATAPI devices. SCSI packets are also commonly exchanged over Infiniband, I20, TCP/IP (iSCSI), even Parallel ports.