THE FIRST MACINTOSH
Steve Jobs of Apple Computer Corporation visited Xerox's Palo Alto Research Center (XEROX PARC), in California and discovered they were running a computer system called the Xerox PARC named for the facility in which it was developed. This Xerox creation was equipped with the very first mouse, a graphical user interface for the operating system called X-Window and a communications system between computers called Ethernet.
Steve Jobs immediately went back and produced the first MacIntosh called the Apple Lisa. The Lisa was a flop. The Lisa was soon followed by the MacIntosh, the Mac Classic, Mac Color Classic, the LC, the Quadras, Centris, Performas, the PowerPC's and the G3, G4 and G5 series systems. To make the MacIntosh computer as user friendly as possible, Apple computer corporation created Mac OS. This OS was intended to be graphically based and very simple to operate.
ENTER OS X
Today, Mac OS is up to version ten and uses the Roman numeral 'X' as its designation. This version is referred to as Mac OS X (OS ten). Mac OS is only available through an Apple authorized dealer or from Apple itself.
MacIntosh operating systems before version 'X' are referred to as the 'classic' operating system. The operating systems use a Major-dot-Minor-dot-Revision (ex: 7.3.1) versioning system. You can upgrade between revisions for free, but any minor or major revision change requires re-purchasing the operating system and performing either an upgrade or clean install of the new OS. OS X programs and OS 9 programs are NOT compatible, however OS X can run OS 9 itself as an application, thereby allowing users to run OS 9 applications within an OS 9 emulation from OS X. Usually, this works, but there are exceptions such as video games.
The Mac operating system is designed exclusively for the MacIntosh line of computers and relies heavilly on a large data store in the ROMs of these computers (which contain the low level graphical user interface functions and appearance).