Windows is a GUI, UNIX is an operating system!
By operating system, I mean the complete suite of programs, which make the computer work.
It is stable, multi-user & multi-tasking system for servers, desktops and laptops.
UNIX systems also have a graphical user interface (GUI), which provides an easy to use environment.
There are many different flavours and versions of UNIX. They share common similarities.
One of the most popular flavour of UNIX for me is Sun Solaris, followed by Linux and MacOS X.