An Operating System (OS) is a program which is responsible for the general operation of the computer. Among other things it controls access to files and applications; error and user instructions; storage on the hard disk and peripherals (e.g. printers and speakers). The saving, loading, deleting, copying and renaming of files are also controlled by the Operating System.

There are a number of different types of OS, including multi-tasking, multi-user and real time.

Multi-tasking – This kind of OS can deal with many different applications being open at any one time. You may be listening to something on Youtube, for example, while writing a letter in Microsoft Word and flicking through your photo album. The RAM of the computer will be switching from one application to another, depending on what demands are being placed on it. Examples of this kind of OS include Windows Vista, Windows 7 and Linux.

Multi-user – A supercomputer or mainframe is a hugely powerful computer which can be used to run very sophisticated programs on it. They may be linked to several terminals. The processing time switches from one user to another, as and when it is required.

Real-time processing – These are normally embedded into something which demands a guaranteed real-time response. RTOSs can be found in cars, planes or even space rockets. The OS will respond to sensors placed across the structure and forms an essential part of a feedback system.