From the Jargon File An operating system is the “…foundation software of a machine; that which schedules tasks, allocates storage, and presents a default interface to the user between applications…”
Linux is a Unix-like operating system that can replace Microsoft Windows or Mac OS. The best thing about it is it’s free. It is available in versions which will run on just about any computer. Most computers sold in the US have Windows or Mac OS already installed, but a few companies are now selling computers with a version of Linux. Apple has recently abandoned their old operating system and created a new one based on Unix but only runs on hardware sold by Apple. There are several varieties of the Windows operating system which will work on computers from many different manufacturers.
Here is one of many places you can go to find out more about Linux .
The Jargon File can also be found here