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 computer science 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.
