By the most simple definition, UNIX ( yoo-niks) is a computer operating system - the base software that controls a computer system and its peripherals. In this sense, UNIX behaves in the same way that the perhaps more familiar PC operating systems Windows or MacOS behave. It provides the base mechanisms for booting a computer, logging in, running applications, storing and retrieving files, etc.
More specificially, the word "UNIX" refers to a family of operating systems that are related to one or both of the original UNIX operating systems - BSD and SystemV. Examples of modern UNIX operating systems include IRIX (from SGI), Solaris (from Sun), Tru64 (from Compaq) and Linux (from the Free Software community). Even though these different "flavors" of UNIX have unique characteristics and come from different sources, they all work alike in a number of fundamental ways. If you gain familiarity with any one of these UNIX-based operating systems, you will also have gained at least some familiarity with nearly every other variant of UNIX.