With the release of Ubuntu 11.10 Canonical has introduced the new desktop systems a graphical environment Unity ,Unfortunately, leaving the majority of Ubuntu users not very satisfied. In fact, a Nuon the users have abandoned Unity to switch to GNOME Shell (Gnome 3), can be easily installed by the official Ubuntu repositories, I said this below describes how to install GNOME Shell in Ubuntu:
Installation using the official Ubuntu PPA repository
open the terminal with the usual key combination Ctrl+Alt+T and type:
sudo apt-get install gnome-shell
we type the root password and confirm the various requests for installation of the package, we will have completed the process of installing Ubuntu both a GUI Unity and GNOME Shell. Below we're going to see how to enable the GNOME Shell just installed!
Enable the GNOME Shell (Gnome 3)
If we have not already done so reboot Ubuntu and let us pause on LightDM or to the login screen as shown below
At this point we click the mouse on the image next to username  and select the voice Gnome  as shown
Then we type our password to login to Ubuntu and carry out in front of us and here is the new graphical environment Gnome Shell (Gnome 3) installed and enabled in the figure below a screenshot of the environment or Gnome Shell.
via : www.lffl.org