The Gnome Website gives a good definition of what the Gnome Shell is supposed to be:
"GNOME Shell is the defining technology of the GNOME 3 desktop 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." (http://live.gnome.org/GnomeShell)
I am pretty excited for how gnome shell is going to look like and how it is going to interact with the user. In my opinion, it is a great approach to a complete different way of using the Desktop - away from the boring, ugly panels like you find them in so many Desktop environments, e.g. Windows, Mac OS X etc.
Gnome 3.0 will also introduce new ways of managing files (with Zeigeist, which is also part of Gnome 3.0) as well as a complete new design to create an innovative new user interface.
Gnome Shell itself is currently in active development, though, you can already try out the preview:
For Ubuntu users, gnome shell is included in the repositories, so using the apt-get tools it can be easily installed:
sudo apt-get install gnome-shell
Once the installation has completed, you can run the shell by typing in the following command:
... which will replace the currently running window manager with the shell.
If you are using a different Linux Distribution, you should check out this Website to get instructions on how the get gnome shell running.
Here are some pictures of how Gnome Shell currently looks like:
(Activities Overview with one desktop and some random programs)
(Activities Overview with multiple desktops. You can add desktops with the "+" button and remove them with the "-" button, which you can see in the bottom desktop on the right hand side)
(You can also zoom into windows when you are in the activities overview by using your mouse scroll wheel - pretty sweet feature and helpful if you have lots of windows open at the same time)
(You probably will never need it, but theoretically you can do this and even more...)