This is a computer's primary software program, and it controls all basic operations of the computer. MS Windows and Linux are examples of operating systems.
The software that bridges the gap between hardware and applications. E.g. Windows XP is the operating system that allows your browser to run on your PC hardware.
The software upon which all other programs run, containing programming routines that are needed to run the computer and otherwise would have to be included in each new program used. PC OSs are dominated by Microsoft first whilst console OSs tend to be proprietary.