The foreword to the SCSI-2 standard draft [ANS] gives a succinct definition of the Small Computer System Interface and briefly explains how SCSI-2 is related to SCSI-1 and CCS:
The SCSI protocol is designed to provide an efficient peer-to-peer I/O bus with up to 8 devices, including one or more hosts. Data may be transferred asynchronously at rates that only depend on device implementation and cable length. Synchronous data transfers are supported at rates up to 10 mega-transfers per second. With the 32 bit wide data transfer option, data rates of up to 40 megabytes per second are possible.
SCSI-2 includes command sets for magnetic and optical disks, tapes, printers, processors, CD-ROMs, scanners, medium changers, and communications devices.
In 1985, when the first SCSI standard was being finalized as an American National Standard, several manufacturers approached the X3T9.2 Task Group. They wanted to increase the mandatory requirements of SCSI and to define further features for direct-access devices. Rather than delay the SCSI standard, X3T9.2 formed an ad hoc group to develop a working paper that was eventually called the Common Command Set (CCS). Many disk products were designed using this working paper in conjunction with the SCSI standard.
In parallel with the development of the CCS working paper, X3T9.2 began work on an enhanced SCSI standard which was named SCSI-2. SCSI-2 included the results of the CCS working paper and extended them to all device types. It also added caching commands, performance enhancement features, and other functions that X3T9.2 deemed worthwhile. While SCSI-2 has gone well beyond the original SCSI standard (now referred to as SCSI-1), it retains a high degree of compatibility with SCSI-1 devices.