An operating system is essentially a program that runs a computer, handling such tasks as managing input/output devices, handling conflicts between applications, allocating CPU time slices to applications, managing shared data, and doing much of the invisible housekeeping behind the applications with which end users normally interface.
There are thousands of different operating systems in existence. Some are widely used in computers and other devices used by consumers and others are special-purpose ones designed for systems that need to be heavily optimized for specific tasks such as satellites or nanotechnology.
The three most common operating systems for personal computers are Windows, Apple OS, and Unix (usually in the form of Linux). Within each of these, several families or generations exist. Unix is still common on many larger computer systems, as are various proprietary systems for supercomputers and parallel computers.