The GNOME Shell redefines user interactions with the GNOME desktop. In particular, it offers new paradigms for launching applications, accessing documents, and organizing open windows in GNOME. Later, it will introduce a new applets eco-system and offer new solutions for other desktop features, such as notifications and contacts management. The GNOME Shell is intended to replace functions handled by the GNOME Panel and by the window manager in previous versions of GNOME. The GNOME Shell has rich visual effects enabled by new graphical technologies.

Install Gnome-Shell

If you have installed Ubuntu-Tweak,you can add the PPA and keep update.

Or,click following link and download a deb package:

https://launchpad.net/ubuntu/+source/gnome-shell

Set gnome-shell as default at start-up

There are three ways and the last one should be the best:
1.Press Alt+F2 and type gconf-editor to go to Configuration Editor,navigate to desktop->gnome->session->required components>windowmanager change its value to gnome shell
2.Start Ubuntu-Tweak –>Session Control–>Window Manager,change its value to gnome shell

3.Go to System->Preferences->Startup Applications,click Add and type gnome-shell –replace in command area