We turn on the computer & perform some work we think that the program is running, mouse is working, keyboard is working, browser & every thing else is working. We think that the computer is taking our command & performing the task as we want them to do or in other word we are controlling the computer & its task. 

Is this true??

NO!!

It is the operating system who makes all these things happen.

An operating system is a computer program that runs a computer and all its other programs

It is a software which organizes and controls hardware and software so that the device it lives in behaves in a flexible predictable manner. 

It actually provides the platform to the hardwares like keyboard, mouse, CD drive, etc to run. It helps the computer to manage information and forms the basic working environment of the system. It is an interface between the computer and its application programs.

OS recognises the input from the input devices operates it & then produces out the output by displaying it on the screen or printing it out etc.It also keeps track of files and directories on the disk, and controll peripheral devices such as disk drives and printers.I does CPU & memory management also.

Whenever we download or install any software, it is the OS who helps us in running it or we can say OS provides platform to run a program or software. 

Let us take a very simple example of a building. We have bricks, cement, labours, sand & everything else, but what is do you think we need most importantly for constructing a building. Its a "PIECE OF LAND". 

Now match the same example with the fact of operating system. We have every thing hardwares, software, a computer, input & output devices, but we do not have an operating system. Then how are we going to use hardwares or run softwares.

Its not only computers that need an operating system but some of the mobile phones with a number of functions in it like Bluetooth, GPRS, etc also requires a n operating system.

We can say that an operating system is nothing but a background which supports the functioning of a device.

Operating system acts as host for the applications running on it.

There are 100s of more operating systems available now-a-days. Here are few of them:- 

It is the most commonly used operating system & it is used worldwide. Microsoft windows has so many versions. Some of the versions are:-

Windows 7, Windows Vista, Windows XP, Windows 2000, Windows 98, Windows 2003, Windows ME, Windows 98, Windows NT, Windows 95 etc.

Macintosh was the most commonly used operating system before the Microsoft Windows.

6. BeOS/ZETA 

7. NextStep 

8. OS/2 Warp/eComStation 

9. Sendla 

10. Amiga 

Even mobile phones use operating system

The popular operating software systems for Smart Phones are 

1. Symbian OS  :- It is the most popular and widely used smart phone OS,

2. Embedded Linux

3. Java (J2ME)

4. Windows Mobile

5. Android