I have a link for this in the links section of this blog but I thought it was worth adding again with a more full explanation about it.
1) Ubuntu is an Operating System (OS), a more common example of an OS is Microsoft Windows, it is a Linux distribution.
2) Ubuntu is completely free, that is free as in it costs nothing and you are free to see how it was made, change it and do anything you want with it! That is to say the source code (how it is written) is available.
3) It ships with approximately 18,000 applications which are also all completely free!
4) You don't have to install it to try it, you can run it DIRECTLY from the CD to see what you think, they'll even send you a CD completely free, you don't even pay the postage!
The not quite so basics:
In the basics we've touched on some reasons for using Ubuntu instead of Windows, i.e. Its free and software for it is free also, there are however several other good reasons for using it:
1) Its fast, and I mean really fast, on a fast computer it absolutely flies but even better if you have an older computer that struggles to run windows XP, chances are Ubuntu will run perfectly on it.
2) Its stable, if you often get lock ups and problems with Windows, such as the dreaded blue screen of death, you rarely if ever get these in Ubuntu.
3) No viruses, no spyware, no adware! Almost all viruses are written for Windows and even if they were written for Linux, because the Source Code is available, the clever people would immediately spot viruses.
4) If you don't play the latest games there is essentially NOTHING you cannot do in Ubuntu and for free! If you do play the latest games (like me) there are ways to get some working in Ubuntu but you can install Windows and Ubuntu like I do.
5) The software available really is excellent, popular software also used on Windows would be Open Office & Firefox, my personal favourites in Ubuntu are Qalculate (A really advanced calculator), Inkscape (Excellent and easy to use for drawing), Listen (Think ITunes but nicer), Gimp (For editing photos) & Audacity (Easy editing of Audio)
6) You have four desktops instead of one and can use them all at the same time!
The fairly advanced:
These things are really good but they're not common to Microsoft Windows so they may sound a little bit strange.
1) The Terminal, this is a place where you can type things in and things happen! You can install applications, run applications, edit files, update your computer, fix your computer, turn it off and anything else you can do in an OS all from the terminal. Doesn't sound very useful but I can promise you that once your used to using it, its hard to be without it!
2) XGL & Compiz, this is a new type of desktop that looks awesome!
That is a video of this in Ubuntu, its German I think but you don't need the audio anyway!
3) www.ubuntuforums.org, if you do decide to install Ubuntu these forums are unbelievably helpful, they'll help anybody out on any question at all!
So thats my basic run down of Ubuntu, i'm completely sold on it and have it set up to do all my work in and windows for when I want to play games! It's great, try it!