GNOME Shell is the core user interface of the GNOME Desktop since version 3.
Upstream consider (quote) that «The GNOME Shell is currently in active development and while many planned features are not yet implemented it is stable enough for everyday use».
Trying it out
First install the shell by opening a terminal window (Applications > Accessories > Terminal) and install the gnome-shell package. This can be done with aptitude using the command:
aptitude install gnome-shell
You should now save all your work in case the Shell fails. Trying the Shell is easy and quick to do, while it may cause you to have to logout (losing any unsaved work in programs you have open) it should not do any harm to your system. To try out the shell open a terminal window (Applications > Accessories > Terminal) and type in:
Press enter and you should now be presented by the GNOME Shell interface. If this does not work it is probably because your computer hardware is not capable of supporting the Shell.
To return to your previous setup just logout and log back in again. To make these changes persistent see Switching to the Shell.
The obvious place to tweak Gnome Shell preferences is Gnome's through "System Setting" (which manage both personal and system setttings in Gnome 3).
The tool gnome-tweak-tool from the package gnome-tweak-tool can help adjust advanced configuration settings for GNOME.
Switching to the Shell
To use gnome shell, either run gnome-shell --replace to replace the window manager in your current session, or choose "GNOME 3" session in your favorite display manager when you open a new session.
The instruction below seems obsolete:
To make the Shell your window manager, so that when you log in you have it straight away just enter this command in to a terminal.
gconftool -s /desktop/gnome/session/required_components/windowmanager -t string gnome-shell
To revert to the metacity window manager just enter this command in to the terminal.
gconftool -s /desktop/gnome/session/required_components/windowmanager -t string metacity