Unix is not an acronym; In 1969 Ken Thompson wrote the first version of Unix, called UNICS. The name “Unix” was intended as a pun on Multics (and was written “Unics” at first — UNiplexed Information and Computing System).
Unix is a powerful, multi-user environment. Unix was developed at Bell Labs in 1969. Examples of Unix implementations include Mac OS X/Darwin (Apple), GNU/Linux, AIX (IBM), Solaris (Sun), IRIX (SGI), and FreeBSD.
Berkeley Software Distribution (BSD, sometimes called Berkeley Unix) is A UNIX-like operating system, a family of Unix versions developed by Bill Joy and others at the University of California at Berkeley. BSD Unix incorporates paged virtual memory, TCP/IP networking enhancements and many other features. BSD pioneered many of the advances of modern computing.
This page lists free and popular operating systems based on BSD (BSD descendants).