Xubuntu is a version of Ubuntu that uses XFCE instead of GNOME as a desktop environment. Typically this means you have a older/slower computer, or just want a very efficient desktop OS. Xubuntu can be run with just 1.5 Gigs of hard drive space and 64-128 MB of RAM.
It is similar to Kubuntu in that it is a derivative of Ubuntu for those who do not want to use GNOME.
External Links
Xubuntu and Beryl