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.
The main part of Gnome Shell is Activities Overview which can be accessed by hovering mouse at top left corner or clicking at “Activities” at top left. Activities Overview have Dash on left side for quick switching and launching of applications, Search bar in middle, Workspace switcher/manager below search bar which is similar to Mac OS X’s Exposé and Application picker.
Additional functionality includes Messaging tray is at the bottom of the screen and notifications pop up in case of any event. Snapping like in Windows 7 when you move windows to screen borders it fill up half or full screen and Fallback mode which have similar interface like in Gnome 2 user interface which is mae up by Gnome Panels.
You can easily install Gnome Shell in Ubuntu 12.04 LTS Precise Pangolin, just run following command in Terminal.
sudo apt-get install gnome-shell
After installing Gnome Shell you should log out and select GNOME as your session for Gnome Shell, to get back to Unity use Ubuntu as you default session in LightDM.