The operating system is the most important program that is on a computer.  The operating system basically runs the computer and allows other programs to run as well.  The operating system does all the basic things that a computer needs to do, such as recognizing inputs from the mouse or the keyboard.  It keeps track of where all the files are on the computer.  It allocates resources to the various programs that are running and it prevents unauthorized access to the computer.

The most popular operating system today is Microsoft's Windows operating system.  Macintosh computers have their own operating system, the most recent of which is called Mac OS X.  There are also open source operating systems such as Linux.