OS X is Apple’s computer operating system, which is pre-loaded on all Apple computers.
Mac OS X is a Unix-based operating system, built on technology developed at NeXT between the second half of the 1980s and Apple’s purchase of the company in early 1996. It received UNIX 03 certification following its 10.5 version on Intel processors.
OS X v 1.0 was released in 1999, with a second, desktop-oriented version released in March 2001. The most recent version is OS X Lion.