An operating system (called an OS for short) is a set of programs that runs your computer. A computer is made of hardware (your monitor, hard disk, motherboard and so on) and software (for example your word processor, and the web browser you are reading this on right now). The job of the operating system is to tell the hardware how it should operate, and how the software should use the hardware. It also provides the basic user interface - the thing that tells you where your files are, how to open them, close them and so on.

You are probably familiar with the Windows operating system, but there are many different types of windows OS, and many other operating systems that are completely different. All the hardware on your computer has a program called a 'driver'. For a piece of hardware to run it needs to have a driver written for the OS of the computer it is plugged into. The driver tells the OS what the hardware is, and how it should be used. Even if the same hardware can be used on several different operating systems, it needs a separate driver for each of them. This means that if you want to change your OS, you need to check carefully that your hardware has drivers for the new OS or thye hardware will stop working.

If an operating system gets into trouble, it might stop working and your computer will crash. Because a crashed windows OS often leaves its final words in white on a blue background, you may hear of people talking about a BSOD (blue screen of death). The important thing required of a modern OS is that it should be stable (i.e. that it should not crash), that it can multi-task (do several different things at once), be powerful (be able to deal with large hard disks, lots of files, and large amounts of RAM) and that it should be secure (so that hackers and viruses cannot damage it). And it should be user-friendly (so that users can do what they want in a simple uncomplicated way).

For many years there were three main operating systems. Windows replaced MS-DOS as the main OS on most desktop computers, while a smaller number of users were dedicated to their Apple computers and their completely different OS. However, more powerful computers which ran networks used a more complex, flexible and powerful OS called Unix. A new arrival called Linux has since enthused computer enthusiasts. Linux is powerful, stable and secure, though many people complain that it is not yet very user-friendly. The interesting thing about Linux is that it has not been written by one person or company, but by hundreds of programmers all around the world who all contribute as individuals. There are many different flavours of Linux, such as Ubuntu, Fedora and SUSE.

In the last few years,'live' versions of Linux operating systems have been distributed on bootable CDs. This means that you can put the CD in your computer, and run Linux from there, so that you can see if you like it before you install it on your computer's hard disk.