Gnome Shell is primary user interface in Gnome 3 but it is not available in Ubuntu 12.04 by default. Gnome Shell is provides unique user interface for switching between windows and launching applications. Gnome Shell uses Mutter (Windows manager) instead of metacity for visual effects and hardware acceleration.
Mate is Gnome 2 fork (Gnome 2 is now not maintained) which enables you to use old Gnome 2 desktop user interface. It is same as Gnome 2 but with renamed packages and applications which lets you install Gnome 2 and Gnome 3 simultaneously on same Ubuntu installation. The Gnome 2 and Gnome 3 desktop [...]
Ubuntu 12.04 LTS beta 2 is released with major Unity changes. After this release final Ubuntu 12.04 LTS Precise Pangolin will be released, so this is the last testing version of Ubuntu 12.04 before final release. Also, now Gnome 3.4 is present in official repositories, so if you want to use cutting edge Gnome-Shell release [...]