MacOS X stands for Mac Operating System 10, and is a series of UNIX-based OS and GUI (graphic user interface) developed, marketed and sold by Apple.
Releases are named after big cats: cheetah, puma, snow leopard, etc.
A version of MacOS X, a POSIX operating system built on top of the XNU kernel, has been released as Open Source and is called Darwin.
As of 2009, MacOS X was the second most popular OS in use for the Internet after Microsoft Windows, with 4.5% of the market share.