Ubuntu is just a linux OS. Like... the windows u might have, its just bttr :P:P (my opinion)

*bit offtopic explanatory (or something) of what ubuntu is

It's one of many Linux versions. Right now it's the best choice for newbies since it's easy to use and has LOTS of support in it's forum, ubuntuforums.org.
If you just want to try it, download the latest desktop version from ubuntu.com, burn it on a cd and start your pc with the cd inside. It will boot from the cd right to the ubuntu desktop, and you can use it right away. It will not touch your current windows setup at all, and when you want to get back to windows, simply shut down ubuntu, remove the cd and boot up again.