OS X is Apple's current operating system for desktop and laptop computers. Use the os-x tag for questions specifically related to the OS X version of a game, or a general feature or issue with the OS. (Use ios for iPad/iPhone/iPod questions)
OS X is the tenth major iteration of the Mac OS series of operating systems, of which the latest current version is OS X Mavericks, released in October 22, 2013.
It is a unix-like OS, meaning it can run most Unix and Linux software. OS X is derived from Darwin, which is derived from NeXT, which is based in part on BSD. Up through version 10.4 (Tiger), Mac OS X retained compatibility for running System 7, OS 8 and OS 9 software (Classic mode ran both Motorola 68K and PowerPC). Version 10.6 dropped support for PowerPC software (native & emulation) and runs exclusively on Intel's x86 architecture.