Mac OS X is a line of proprietary, graphical operating systems developed, marketed, and sold by Apple Inc., the latest of which is pre-loaded on all currently shipping Macintosh computers. Mac OS X is the successor to the original Mac OS, which was Apple's primary operating system from 1984. Unlike its predecessor, Mac OS X is a Unix-like operating system built on technology developed at NeXT from the second half of the 1980s until Apple purchased the company in early 1997.
The operating system was first released in 1999 as Mac OS X Server 1.0, with a desktop-oriented version (Mac OS X v10.0) following in March 2001. Since then, four more distinct "end-user" and "server" editions of Mac OS X have been released.
Mac OS X was a radical departure from previous Macintosh operating systems; its underlying code base is completely different from previous versions. Its core, named Darwin, is an open source, Unix-like operating system built on top of the XNU kernel, with standard Unix facilities available from the command line interface. Apple layered over Darwin a number of proprietary components, including the Aqua interface and the Finder, to complete the GUI-based operating system which is Mac OS X.
Mac OS X included a number of features intended to stabilise and increase its reliability compared to Apple's previous operating systems. Pre-emptive multitasking and memory protection, for example, improved the ability of the operating system to run multiple applications simultaneously without them interrupting or corrupting each other.
http://en.wikipedia.org/wiki/Mac_OS/OS_X Accessed: 17 April 2007