Ubuntu is full version desktop using Gnome while Xubuntu is a "lightweight" desktop using Xfce which is more stripped down version with minimal look. You can install KDE desktop using synapic package manager.
Xubuntu is good for old PC or laptop with limited RAM memory while Ubuntu is good for any PC 4 years old or newer (I have 6 years old system and It ran great on it with 1GB ram and Nvidia graphic card to top it off).
There are Edubuntu too which is configured for educational use.
There are some other distros that use Ubuntu's kernel such as Mint Linux, Crunchbang Linux, and several more.
I have been hopping to many different distros, Now that I can tell which kernel originally came from.
Catty