UNIX is not one single operating system, it is a family of operating systems. Different computer manufacturers produce their own versions of UNIX. Although these are mostly similar, there are small differences which can cause problems. The most obvious examples are the layout of the file system and the exact format of certain commands.
The first version of UNIX was created in 1969 by Kenneth Thompson and Dennis Ritchie, system engineers at AT&T's Bell Labs, in an effort to provide a multiuser, multitasking system for use by programmers. The philosophy behind the design of UNIX was to provide simple, yet powerful utilities that could be pieced together in a flexible manner to perform a wide variety of tasks. It went through many revisions and gained in popularity until 1977, when it was first made commercially available by Interactive Systems Corporation.
At the same time a team from the University of California at Berkeley was working to improve UNIX. In 1977 it released the first Berkeley Software Distribution, which became known as BSD. Over time this won favor through innovations such as the C shell.
Meanwhile the AT&T version was developing in different ways. The 1978 release of Version 7 included the Bourne Shell for the first time. By 1983 commercial interest was growing and Sun Microsystems produced a UNIX workstation. System V appeared, directly descended from the original AT&T UNIX and the prototype of the more widely used variant today.