Mac OS X is an OperatingSystem for the Macintosh computer. It uses the Mach MicroKernel which provides InterProcessCommunication and driver support for the Darwin environment and the Classic environment (which run as servers on top of the MicroKernel). Basically, it's UnixOs.
The GUI used in Mac OS X is based on NeXT, and all programming is done with ObjectiveCLanguage. It is often referred to as "lickable", because everything is shiny, round, and blue, like candy.
Because of this, Mac OS X has made the Macintosh platform exteremly appealing to geeks. Not only does it have the power and flexibility of UNIX, it also looks nice and JustWorks. There is no messing around with configuration files, unless you're really into that kind of thing.
There is a lot of effort put towards porting UNIX tools to Mac OS X; most UNIX geeks with a Mac should check out fink.
The beauty of this particular operating system is that is the first Mac operating system to break away from the traditional design and interface, which, to many users, is extremely ugly. Rather, it has been completely remodelled to appear far more ethereal, featuring glass-like buttons and tabs, which renders it much easier to use.
The upshot of this is that Mac OS X is now infinitely more stable and powerful than its predecessors, and is a very popular operating system indeed, even among former users of other OSs (such as WindowsOs).