Unity 2D is a lighter version of the normal Unity (or Unity 3D) which comes with Ubuntu 11.04. Unity 3D does not run in systems which does not support 3D acceleration. Unity 2D will however run on such systems and it is also a lot more responsive compared to Unity 3D.
You can have both Unity 2D and Unity 3D installed on the same Ubuntu installation and select which one to launch at the login screen. To install Unity 2D, open the Terminal and execute the following commands:
$ sudo add-apt-repository ppa:unity-2d-team/unity-2d-daily$ sudo apt-get update$ sudo apt-get install unity-2d
