When it comes to computers operating systems are basically the brains of the operation. In fact, they are the brains behind every operation. Your operating system does just about everything from getting input from your mouse and keyboard to accessing data stored throughout your hard drive.
Operating systems are responsible for running the tasks that the applications require.
For example the text you are reading right now was not put up on the screen by your browser but by your operating system. The browser tells the operating system what needs to be done the operating system is what does it.