Where did UNIX come from and why are there different versions of UNIX?

The first efforts at developing a multi-user, multi-tasking operating system were begun in the 1960's in a development project called MULTICS. While working for Bell Telephone Laboratories in 1969 and 1970, Ken Thompson and Dennis Ritchie began to develop their own single-user, multi-tasking small operating system and they chose the name UNIX. Their initial goal was simply to operate their DEC PDP machines more effectively. In 1971, UNIX became multi-user and multi-tasking, but it was still just being …

…System, their parent company, AT&T, became much more interested in marketing a commercial version of UNIX. And today we see that many companies have now licensed their own version: AT&T's System V, Versions of System V such as SCO's Xenix and IBM's AIX Berkeley's UNIX (called 'BSD' for 'Berkeley System Development'), Versions of Berkeley UNIX such as Sun Microsystem's SunOS, DEC's Ultrix and Carnegie Mellon University's Mach (used on the NEXT).