OS X (pronounced "o-ess-ten"; originally Mac OS X) is a series of Unix-based graphical interface  operating systems (OS) developed and marketed by Apple Inc. It is designed to run on Macintosh computers, having been pre-installed on all Macs since 2002. OS X is the fourth most popular general purpose OS; within the market of desktop, laptop and home computers, and by web usage, OS X is the second most widely used desktop OS after Windows.