Unix is a family of related or similar multi-tasking operating systems originally developed by Bell Labs in the late 1960s and 1970s for mini-computers, such as the PDP-11. Unix was developed by C programmers, probably because C was also developed by Bell Labs. "Unix has never really made any inroads on the desktop."
Over time improvements to Unix were developed by others under license from AT&T. They used the AT&T source code but generally did not use its Unix trademark. Important examples from the 1980s include BSD, SunOS and Xenix.
In the 1990s AT&T sold Unix to Novell. Novell transferred the trademark to X/Open in 1994, which in turn licensed it for use on any operating system which met the Single Unix Specification, separating the name "Unix" from the original Unix source code. X/Open is now part of The Open Group, having merged with The Open Software Foundation.
Novell later sold its business of licensing and supporting System V Unix (the direct descendant of the original AT&T Unix) to The Santa Cruz Operation, commonly called "SCO"; exactly which rights were sold, and whether the copyright itself was sold, is part of the dispute in the SCO v. Novell lawsuit. In 2000, Caldera Systems purchased the server software division of SCO, which then changed its name to Tarantella. In 2002, Caldera changed its name to The SCO Group.
In 2003, SCO filed lawsuits against IBM, Autozone and Daimler-Chrysler for using Linux, an open source clone of Unix developed for the PC by Linus Torvalds and others over USENET. Simultaneously, SCO declared it was entitled to a license fee of $699 for every copy of Linux in use anywhere. Novell in turn sued SCO for slander of title and ordered SCO to drop the suits. All of these court actions are still moving through the federal legal system in Utah. If SCO wins, millions of Linux users will be liable for licensing fees, potentially bankrupting all but the wealthiest users.
Today, the main AT&T branch of Unix is nearly irrelevant, having been eclipsed by the Unix-like Linux operating system and the various BSD derivatives. Sun's OpenSolaris and HP's HP-UX, and Mac OS are the only offshoots of SVR4 still in development and widespread use.