This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
SCSI is the Small Computer System Interface. It is standardized by ANSI X3T9.2. It is mostly aimed at storage devices, with command sets defined for disks, tapes, and autochangers, but also includes communications devices, printers, and scanners. It's daisy-chained, with a maximum of eight devices (including the host computer) on a single narrow bus (there are non-standard schemes for 16 devices on a wide bus). Any device can be an initiator, so it's possible to use the bus for sharing devices between hosts, provided your software can manage it. See also the newsgroup comp.periphs.scsi, especially for "How do I hook up a Brand X diskdrive to my Atavachron 9000 PDA?" type questions. There is also an FTP site for some working documents for the SCSI-3 committees and other X3T10 documents. See ftp://ftp.symbios.com or ftp ftp.hmpd.com. You'll find good info at http://www.symbios.com/x3t10/ and at http://www.scsita.com.