Mac OS
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The original version was the integral and unnamed system software first introduced in 1984 with the original Macintosh, and referred to simply as the “System” software. The System was renamed to Mac OS in 1996 with version 7.6. The System is credited with popularizing the graphical user interface concept.

OS X:

OS X, previously Mac OS X, is a series of Unix-based graphical interface operating systems developed, marketed, and sold by Apple Inc. It is designed to run exclusively on Mac computers, having been pre-loaded on all Macs since 2002. It was the successor to Mac OS 9, released in 1999; the final release of the “classic” Mac OS, which had been Apple’s primary operating system since 1984. The first version released was Mac OS X Server 1.0 in 1999, and a desktop version, Mac OS X v10.0 “Cheetah” followed on March 24, 2001. Previous releases of OS X were named after big cats; for example, OS X v10.8 was referred to as “Mountain Lion”. However, with the announcement of OS X Mavericks this was dropped in favor of Californian landmarks.
Reference
Wikipedia
Operating Systems
Android OS
Mac OS
Windows OS