Linux is a UNIX-clone.
It was created by Linus Torvalds in 1991 as an alternative to Minix. Since then, many programmers around the world shaped Linux into a modern operating system. Most commonly today found as GNU/Linux. Linux is written using mostly the C programming language.
If you want to try Linux without affecting your computer permanently, all you need is to obtain a "live-on" cd such as Knoppix. You can either download it from their site and burn it, get a friend to, or buy it. Then when you boot off this cd-rom it will load the image into RAM. It won't delete anything you have on your harddrive. At any time you can decide you want to just got back to the operating system on your hard drive by taking out the cd, or you might want drop knoppix from cd onto your harddrive so it is a lot faster and more customizable. Or you might want to move onto a common distribution.
If you have a spare computer, or are ready to move to linux, you can download and install any of the following for absolutely free. Common Distributions include the following:
Handy Documentation can be found here: