MacOS is a generic term for several generations of MacIntosh (or iMac) operating systems. The current Apple operating system (MacOS 10) is a complete departure from earlier versions and is known as OS X.
OS X is a proprietary Apple GUI running on the Darwin BSD kernel, compatible with Power Macintosh G3 "beige box" and later platforms. Not only is it a better OS than that one from Redmond, but you can get free open source software for it to do just about anything. The big cats are release versions.
