It's a LINUX Operating System. A good one. Better than XP, Vista and whatsoever coming out by Microsoft because it is VERY customizeable and most important, it is open source (No need to pay for license).

But i must warn you... It's completely different from Windows. So, if you are new to Ubuntu, u might find it a little difficult to use. But hey, it worth to learn. Really it is...

more info here
http://www.ubuntu.com/