BESYS and Multics were two ancestoral inspirations for UNIX.
UNIX was one of the first computer operating systems that was written in a language suitable for cross-platform development.
Much of UNIX's early development occured inside of places such as Universities and research centers.
During the mid-1980s, AT&T (the owner of UNXI) radically altered their permissiveness with respect to UNIX and began selling it to third parties that, in turn, began selling their versions of it back to the people who had been developing on and for it.
Because of this, groups such as The Free Software Foundation (AKA, the GNU Group) were formed to produce Free (as in "Freedom") replacements for the proprietary UNIX variants.
BSD
While UNIX development continued at Bell Labs, Ken Thompson (if you'll recall, he was one of the original creators of UNIX) visitted the University of California at Berkley in the fall of 1974 to teach for a year. He brought with him his UNIX, and Berkley quickly latched onto it, using it and its source code in the classroom. A group was formed for the production of their own "flavour" of UNIX, called BSD (Berkley Software Distribution) and they released their first version in 1978. This version was based upon the original UNIX code that AT&T licensed to them.
When AT&T altered their distribution policies of UNIX and released System V, the Berkley developers continued to develop and enhance their BSD UNIX. Here is where we see the first fork in the history of UNIX: The System V developers had a very "commercial" thrust to their UNIX while the BSD developers had a very "grass-roots" goal for theirs. When writing about this dichotomy, Eric S. Raymond said "The divide was roughly between longhairs and shorthairs; programmers and technical people tended to line up with Berkeley and BSD, more business-oriented types with AT&T and System V."
By the time that RMS began work on his GNU project, there was already a slightly free (as in cost) UNIX distribution in the form of BSD. However, BSD contained much code from the original AT&T source, so several developers set about to remove this source (actually spurned on by a request from RMS).
However, the selling of System V UNIX to vendors began what became known as the "UNIX Wars", where vendors and distributors of UNIX and BSD became wrapped up in squabbles and conflicts, all the time ignoring the growing low-end system market (led by low-cost IBMs, and eventually Intel-based machines).
By 1989, the 386 Intel-class processor family had been in production for a few years and was already capable to compete with UNIX processors on the low-end. Developers at BSD had attempted to port it to this class of processors (calling it BSDi), but they were having mixed success, and the UNIX Wars had made the Unix vendors blind to the growing threat of low-end machines and Microsoft's operating system.
About these times, Eric S. Raymond (ESR) writes:
The years from 1989 to 1993 were the darkest in Unix's history. It appeared then that all the Unix community's dreams had failed. Internecine warfare had reduced the proprietary Unix industry to a squabbling shambles that never summoned either the determination or the capability to challenge Microsoft. The elegant Motorola chips favored by most Unix programmers had lost out to Intel's ugly but inexpensive processors. The GNU project failed to produce the free Unix kernel it had been promising since 1985, and after years of excuses its credibility was beginning to wear thin. PC technology was being relentlessly corporatized. The pioneering Unix hackers of the 1970s were hitting middle age and slowing down. Hardware was getting cheaper, but Unix was still too expensive. We were belatedly becoming aware that the old monopoly of IBM had yielded to a newer monopoly of Microsoft, and Microsoft's mal-engineered software was rising around us like a tide of sewage.
