Its an Operating System based on Linux. Its free – you can download it now, install it and replace Windows XP/Vista/7 or use it in conjunction with Windows – as I do. Ubuntu has a massive user base, a huge base of supported applications and many of these are also free, released under the same license as Ubuntu itself. Ubuntu has also made new and successful efforts to bring Linux to the Desktop. A very impressive project.
In time I will write some guides to install it and use it and post them here.