Lol, without some sort of OS we can not do anything with computers.. they might as well be a pile of scrap metal. The operating system is how everything in a computer talks to each other. Check out this: http://en.wikipedia.org/wiki/Operating_system
A good way to put it would be to say that the operating system is a template for a computer and you use this and put whatever you like on it aslong as you don't run out of space and you can power it. (which is where the hardware comes in to it)