Xubuntu is a community-maintained derivative of the Ubuntu operating system using Xfce desktop environment, instead of Ubuntu's Unity.
Ranked in these QuestionsQuestion Ranking
Pro Comes with lightweight applications
Xubuntu is designed as a lightweight operating system and Linux distro. It uses XFCE by default and comes packed with XFCE applications which are generally pretty lightweight.
Pro Great performance
Xubuntu is very fast and makes good use of resources.
Pro Quick boot time
Boots at about double the speed of most distributions.
Official distro of XFCE, one of the most customizable desktop environments. In XFCE you can create as many tasks bars as you need and configure every one of their elements and behaviours. You can also change any icon, font, color... etc. Literally there's nothing you can't change in GUI.
Pro Conservative, yet modern
It comes with all Ubuntu goodness but without bloat, a perfect mix between new ideas and usability.
Con XFCE development is slow
The desktop environment used in Xubuntu has not been updated in over two years and is developed at a snail's pace.
Con UI is pretty generic without customization
The default XFCE UI is pretty generic and sometimes ugly unless customized to suit the user's needs and liking. Customization in XFCE is easier than on Unity, but still hard for beginners.
There is absolutely no reason to install it, since it uses the same package repositories as Ubuntu and you can transform any Ubuntu install into a Xubuntu install.