UNIX is a general-purpose, multi-user, interactive operating system for the Digital Equipment Corporation PDP-11/40 and 11/45 computers. It offers a number of features seldom found even in larger operating systems, including:

1. A hierarchical file system incorporating demountable volumes;
2. Compatible file, device,and inter-process I/O;
3. The ability to initiate asynchronous processes;
4. System command language selectableon a per-user basis;
5. Over 100 subsystems including a dozen languages.

In 1969 AT&T withdrew from a collaborative project with the Massachusetts Institute of Technology and General Electric to create an interactive time-sharing system called Multics, which stood for "Multiplexed Information and Computing Service."
As a lark with no official backing two of the developers, Ken Thompson, Dennis Ritchie, decided to work on their own Operating System, calling it Unix as a parody of Multics.

In a 2013 Forbes article Is Unix Now The Most Successful Operating System Of All Time?, they point out that UNIX variants (iOS and Android) on smart phones and tablets adds up to 900 million concurrent users. That doesn't count BSD UNIX on the Mac OSX OS and Linux systems both commercial and on PCs.
There are 1.2-1.4 billion PC's. As of May 2013 there were 450 million Windows 7 copies and 100 million Windows 8.

UNIX variants like Sun Microsystems Solaris, Linux and Linux derivatives Red Hat, SUSE, and Ubuntu have proliferated.
It runs on everything from PCs to mainframes.

Some history:
1970 Ken Thompson and Dennis Ritchie and others develop UNIX at Bell Laboratories
1970's A variety of UNIX versions proliferate within Bell Laboratories
   The research version at Murray Hill, NJ.
   The version (PWB/UNIX) for developing operations support systems at Piscataway, NJ, 
   which included programers workbench and writers workbench.
   Another version at Columbus, OH
1977 The University of California, Berkeley releases BSD UNIX  (Berkeley Software Distribution)
     Colleges and Universities used to buy UNIX from Western Electric
     (The manufacturing arm of AT&T, which owned Bell Labs,
     so they had a legal copy, then send to tapes to Berkeley to get BSD.
late 70's AT&T tried to commercialize it by licensing the OS to third-party vendors,
    leading to a variety of both academic (e.g., BSD) and commercial variants of Unix
    (such as Xenix) and eventually to the "Unix wars" between groups of vendors.
1984 X/Open Company, Ltd founded by a consortium European UNIX systems manufacturers
     to  identify and promote open IT standards, particularly
     a single specification for or operating systems derived from UNIX,
1984   The GNU Project formed to develop a free software system, upward-compatible with Unix.
      The name "GNU" is a recursive acronym for "GNU's Not Unix!"
198? X/Open Portability Guide (XPG) Issue 1 published by X/OPEN
1987 In a move intended to unify the market, AT&T announced a pact with Sun Microsystems,
      the leading proponent of BSD UNIX
1988 Efforts to establish a family of standards for computer operating systems
     to qualify for the name "Unix" a Single UNIX Specification (SUS) by
     IEEE and ISO resulted in IEEE 1003 or POSIX 
     (Portable Operating System Interface for uniX)
1990 AT&T reorganized its business and transferred title to UNIX System Laboratories, Inc. (USL),
     a new wholly owned subsidiary.
1993 UNIX System V - AT&T sells UNIX rights to Novell
     Dennis Ritchie likened this sale to the Biblical story of Esau selling his birthright.
     Novell transferred the trademarks of Unix to the X/Open Company (now The Open Group).
     Many employees of Unix Systems Labs were offered jobs at HP (Hewlett Packard) to work on HP-UX
1995 Novell sells UNIX to Santa Cruz Operations (SCO)
     HP, Novell and SCO announce business relationships designed to deliver
     a high-volume UNIX operating system with NetWare and UNIX enterprise services. 
2001 SCO sold the SCO brand, SCO OpenServer (SCO's version of UNIX),
      and the Bell Labs version of UNIX to Caldera,
      which now does business under the SCO name.