Operating systems tell the devices that make up your Pc how to work together. The operating system is the method that your programs use to be able to use the different devices on your PC such as the printer, keyboard, monitor and mouse.
If we did not have an operating system them you would have some nice computer equipment but all you would be able to do is look at it but it would not do anything for you. When you start the PC the operating system determines what all equipment you have and how to communicate with them. The operating system then manages the programs running and the various devices.
When you start Internet Explorer you tell the operating system to go and get the program and to run it. Then the operating system sends the out\put from the program to the screen and goes and gets stuff from the network to give to Internet Explorer to use to properly display web pages you want.
There are lots of operating systems in the world. The three most common groups of operating systems currently are Windows, MAC OS and Linux. There are different versions of these operating systems. For instance you may be using Windows XP, Vista or 7. Windows 8 will be available next year and promises a friendlier interface and ability to use newer devices efficiently
New versions of operating systems come out regularly. New operating systems are needed as they give abilities to work with new devices, new programs and better ways for us to interface with the operating system (DOS just used a command prompt but Windows gives a friendlier graphical user interface.)
In just the last few years we have moved to many of our devices using USB connections and our hard drives have gotten much larger.
The operating system also provides security for your computer and allows you to block items and people from harming your computer.
The operating system is critical for your PC just as you need a driver for your car can be useful.