In short a Operating system is the program that your pc runs off.

If you want to be technical.

An operating system is software, consisting of programs and data, that runs on computers, manages computer hardware resources, and provides common services for execution of various application software. The operating system is the most important type of system software in a computer system. Without an operating system, a user cannot run an application program on their computer, unless the application program is self booting.
For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between application programs and the computer hardware,although the application code is usually executed directly by the hardware and will frequently call the OS or be interrupted by it. Operating systems are found on almost any device that contains a computerfrom cellular phones and video game consoles to supercomputers and web servers.
Examples of popular modern operating systems are: BSD, Linux (Ubuntu, Fedora, OpenSuSE, Debian etc.), Mac OS X, Microsoft Windows and UNIX.

[ Register or Signin to view external links. ]

Basically without and operating system your PC/Laptop would be near enough useless.
This is the base of your PC.
Think of it like the foundations of a building.

Windows
XP/Vista/7

Mac OS X

Linux

There are many others but few people use them.
These are the main 3. You are probably using one of the right now reading this.

Video's













Text Tutorials.

1:
Back up all user files if you're planning to perform a system recovery. Buy a large stack of CDs or multiple flash drives. This is the best way to back up your hard drive, especially if you've got a large quantity of files which take up a lot of space on the drive. The flash drives are particularly useful if you are unable to burn files to disk. You can use the flash drives to transfer files to another computer, and then burn them to CD. You also might want to try zip drives. Other alternatives include connecting the computer to another computer using a null modem cable or a networking USB cable, then downloading all user files to the second computer. Also, an external hard drive works very well as a backup drive, as does a tape drive. This step is primarily for user files which you and other members of your household created. Save important information such as program activation codes, your internet access information and usernames and passwords.

2:
Gather together all the installation CDs which came with your computer and with the devices you have added to your computer. This may include your printer, modem, router, access point, disk drives, graphics card, sound card and any other devices which have their own separate installation software. Create a boot disk for your chosen operating system to guard against system failure.

3:
Ensure that your computer is set up to boot from the CD-ROM drive. This is done in the system BIOS. Most computers access the BIOS by selecting a function key on boot-up. Watch the splash screen as your computer boots, and select the function key which your screen instructs to get into setup. In the boot menu of the BIOS setup, select the CD-ROM drive as the first choice for booting the computer.

4:
Place the first CD for the operating system in the CD drive. This will either be the first CD of a set of recovery CDs, or a standalone version of the operating system on CD.

5:
Turn the computer off. Wait a full 30 seconds before turning it back on again. Turn the computer back on, and follow the instructions on the screen to partition and format your hard drive. This will erase all data which previously existed on the drive.

6:
Follow the instructions which appear on the screen. Everything should be smooth sailing from this point forward. However, it's a good idea to babysit the installation to be ready for any problems which might crop up, and to click into the next step when the installation program prompts you. You will also probably need to check your speaker connections to ensure that they are properly connected.

7:
Reconnect to the internet. Troubleshoot any problems with your installation by visiting the software provider's support website. Look in particular for online knowledge bases. These contain valuable information which can help you to overcome even the most difficult installation problems. Use several different search terms to search for the solutions you need.

8:
Install or reinstall your antivirus software and promptly download and install all the definition updates. If possible do this without opening a web browser.

9:
Navigate immediately to the update site of the operating system's manufacturer and download all critical updates. Set up automatic updates if possible. Don't forget to upgrade the browser to the latest version, as old browsers often have security vulnerabilities which are susceptible to viruses and hackers. Avoid beta versions of web browsers.

10:
Install the software for any additional devices which did not come with your computer. Update the drivers for all of your devices. Check device manager to see which version of each driver you are using and check the date.

[ Register or Signin to view external links. ]

Video's










Text


1:
Make a copy or backup of important files or programs before proceeding. Saving those important files and folders allows you to access them later on, when you load a new operating system.

2:
Install the new operating system. This is probably the easiest and fastest way to remove or delete your current operating system. Insert the new operating system CD and follow the installation prompt. If it asks you if you are sure you want to delete the current operating system, click on \"Y\" or \"Yes.\"

3:
Remove the Windows operating system completely by going into \"Safe Mode.\" Try hitting the F8 key repeatedly after the initial bootup. Using the arrow keys, navigate to the Command Prompt (DOS), then hit the \"Enter\" key. When you get to C:\\<WINDOWS>, type "cd" without the quotation marks, then hit the \"Enter\" key again. You will now be in your local drive directory, or C:\\, where you can type the command DEL WINDOWS (note the space between the words). Then hit the \"Enter\" key again. Click on \"Y\" when it asks if you are sure you want to delete your hard drive. This will then proceed to formatting your hard drive, which deletes the operating system.

4:
Install the new operating system using an installation or bootup CD. Follow the prompts to completion.

5:
Install the other drivers, such as sound card, video card and CD-ROM.