Linux
This is an open-source operating system that is available freely to anyone. It id based on UNIX. It was originally developed by Linus Torvalds with the help of many developers all over the world.

Mac OS X
Mac OS X is the latest version of the Mac OS, the operating system software for Macintosh computers. A realistic alternative to Microsoft Windows, but has failed to knock it off the top spot.

Windows
Created by Microsoft in 1985 as an add-on to MS-DOS. Microsoft Windows is a family of operating systems, ranging from Windows 95, tot the current Windows XP and the soon-to-be launched Microsoft Vista.  Windows is proffered by most personal computer users, and according to thecounter.com about 93% of computer users use Microsoft windows.