Xubuntu, pronounced as Zooboontoo, is a complete GNU/Linux based operating system with an Ubuntu base. It is lighter on system requirements and tends to be more efficient than Ubuntu with GNOME or KDE, since it uses the Xfce Desktop environment, which makes it ideal for old or low-end machines, thin-client networks, or for those who would like to get more performance out of their hardware.
Xubuntu is shipped with top quality packages from the Ubuntu archives. Xubuntu will use, as far as possible, GTK2 applications, ensuring maximum efficiency. If you wish, you can install any KDE or GNOME application on your Xubuntu desktop system, as it shares the Ubuntu main and universe package selection.
Ubuntu is a free, open source Linux-based operating system that starts with the breadth of Debian and adds regular releases (every six months), a clear focus on the user and usability (it should "Just Work", TM) and a commitment to security updates with 18 months of support for every release (and with 6.06 LTS you get 3 years on the desktop and 5 on the server!). Ubuntu ships with the latest GNOME release as well as a selection of server and desktop software that makes for a comfortable desktop experience off a single installation CD