If you have a computer, then you have probably heard about operating systems. Any desktop or laptop PC that you buy normally comes pre-loaded with Windows.
Windows is an operating system from Microsoft, and currently has over 90% of the market share worldwide for personal computers.

Windows was first introduced by Microsoft in 1983 to replace their DOS operating system.
Some of the versions of Windows include: Windows 3.1, Windows NT, Windows 95, Windows CE, Windows 98, Windows 2000, Windows 2000 Server, Windows ME, Windows XP, Windows Server 2003, and Windows Vista.

Macintosh is the operation system from Apple. Apple computers currently come pre-loaded with the OS X operating system.
The first Macintosh from Apple was released in 1984. The current version, Mac OS X Tiger (10.4), uses a UNIX foundation for its kernel.

Linux was developed by Linus Torvalds, a graduate student from Finland, in 1991. Linux is a small and efficient operating system based off of the UNIX model. Linux is currently the dominant operating system used in servers, for large corporations globally.

In 1969, Ken Thompson and Dennis Ritchie from AT&T's Bell Labs created the UNIX operating system. The UNIX operating system is widely used for servers by large corporations.