Xubuntu

Xubuntu (Pronounced: /zù'búntú/) is an official Ubuntu-based operating system. Xubuntu uses the Xfce desktop environment because it works well on older machines. Xubuntu is intended for use on less-powerful computers, or those who seek a highly efficient desktop environment on faster systems. It mainly uses GTK+ applications.

Basic Summary
Xubuntu is a fully functional, stable, Xfce based operating system that can run on nearly any computer.

Differences from Ubuntu
Xubuntu, although based on Ubuntu, is a somewhat different operating system.
 * Ubuntu uses the GNOME Shell desktop environment, while Xubuntu uses Xfce
 * Xubuntu runs more efficiently on older hardware
 * Xubuntu and Ubuntu include different applications to fill the same role (ex. Thunar vs Nautilus)
 * Xubuntu features GTK+ apps that are designed to use less resources than their GNOME counterparts

History
The first Xubuntu release appeared on 2005, alongside the Ubuntu 5.10. It was avalible only to install package "xubuntu-desktop".

The first official Xubuntu release appeared on June 1, 2006, alongside the Ubuntu 6.06 line (including Kubuntu and Edubuntu) code named Dapper Drake.

System Requirements

 * 512MiB for i386, 1400MiB for x86_64.
 * 5-7 GB of free disk space.

xubuntu-desktop Package
In Synaptic, you can install the xubuntu-desktop package. This will install everything that Xubuntu has by default onto your Ubuntu (or other Ubuntu derivative) system without needed to completely reinstall your system.