Mac OS is the OS used on Macintosh computers. There are two distinctively different versions of it; everything prior to version 10 (sometimes called Mac OS Classic) and everything version 10 or later (called Mac OS).
The Macintosh Operating System, or software that makes your Macintosh work. It is sometimes referred to as the "System Software". The current version of Mac OS is 7.5.2. It is pronounced "MAK-OH-ESS", although some people pronounce it "MAK-OSS". The operating system of a computer is the software that runs atop the hardware (i.e., RAM, Hard Drive, and Logic Board) and allows you to run application software to accomplish work on the computer.
The operating system used on Apple's Macintosh and iMac computers. Many find it easier to use than Windows, but it requires programs designed specifically for the Macintosh. The latest version is Mac OS X.
The Macintosh Operating System. The control program that governs the way the Macintosh handles files and applications internally, interacts with other hardware and software, and allocates resources like memory.
Mac OS, which stands for Macintosh Operating System, is the trademarked name for a series of graphical user interface-based operating systems developed by Apple Computer for their Macintosh line of computer systems. The Mac OS is often credited with popularizing the graphical user interface. It was first introduced in 1984 with the original Macintosh 128 K.