GNOME

Gnome is the GNU Network Object Model Environment. It intends to build a complete, easy-to-use desktop environment for the user and a powerful application framework for the software developer.

Translation: It's like the Windows desktop, but for UNIX, GNU/Linux etc.

if you want more information check out http://www.gnu.org/directory/gnome.html but most GNU/Linux users already have it, and most Windows users wouldn't really need it...