For those of you that don't know, an operating system is the computer program that runs all the OTHER computer programs. Windows is actually a program, and it's ridiculously expensive. The fact that it comes on your computer is no different than the fact that AOL came on your computer 10 years ago. They're both just programs. The Operating System is the basic level of operations that you can access, and it controls how the other programs run and which of your computer's resources are directed to which programs. Mac OSX is another famous operating system.