The difference between Gnome Shell and Unity in Ubuntu

I was thinking about the difference between Gnome and Gnome Shell. The screenshots of Gnome that I have seen on their website were different of what Gnome I got in Ubuntu 11.10 with Unity.

I found Gnome Shell is not installed on my Ubuntu 11.10. I upgraded from Ubuntu 11.04. Why was it like that? The reason is, Gnome Shell is not shipped with Gnome 3.

tahir@StoneCode:~$ gnome-shell --version The program 'gnome-shell' is currently not installed. You can install it by typing: sudo apt-get install gnome-shell

And why I need Gnome Shell? When I am using Unity. I have seen many people who are not using Unity and are using installed Gnome Shell. Actually I figured it out, Gnome Shell is a separate component. And you can not use Gnome Shell when you are using Unity interface. If you are happy with Unity interface, then there is no need to use Gnome Shell. Gnome Shell means a different UI than Unity. It’s like a Gnome Classic.

If you guys are interested to use Gnome Shell, you can study this guide.