Mac OS X, like Ubuntu and other Linux distributions, belongs to the Unix-like family of operating systems. Specifically, it is based on Darwin, an open-source operating system also developed by Apple; unlike Darwin, however, Mac OS X is closed-source and proprietary. It is the successor to Mac OS 9.
Mac OS X is only available on computers produced by Apple. It is currently the second most-popular desktop operating system, after Windows and before Ubuntu.