Ubuntu 11.04 Natty Narwhal released almost one and a half years ago will end support today. This comes according to the policy of drastic six months of OS upgrades and a support for one and half years for each. Long term support Ubuntu releases have a greater support period, extending upto five years for Ubuntu 12.04. These long term support releases are more suitable for business and enterprise environments can can be used in servers and workstations as the main OS.
Ubuntu 11.04 first debuted the Unity interface. The shift of Gnome development from Gnome to the then new and controversial Gnome 3 made Canonical developers to make their own version of desktop environment. This DE was based on Gnome 3 and used Compiz as the window manager. So users can also enjoy the amazing compiz effects and plugins and Emerald themes. Nevertheless, Unity was still experimental and buggy and most users hated it.
From them a lot of improvements have been made in the Unity desktop and it is quite usable. If you are still on Ubuntu 11.04, upgrading will be a wise option.