GNOME Shell is the defining technology of the GNOME 3 user experience. It provides core interface functions like switching to windows and launching applications. GNOME Shell takes advantage of the capabilities of modern graphics hardware and introduces innovative user interface concepts to provide a delightful and easy to use experience.
Install Gnome Shell in Ubuntu 12.04 Precise Pangolin/Ubuntu 11.10 Oneiric Ocelot
Open the terminal and run the following command
sudo apt-get install gnome-shell
Now you have to logout from the existing session if you want to login in to gnome shell desktop you need to select GNOME from the login Screen.