Gnome Shell is a redesign to the way the gnome desktop looks and works. I find it very interesting, because it stops trying to base itself on windows' functionality and design (although gnome isn't exactly like windows' desktop, it had to many similarities to not have been based on it), and instead it consists of an original design and functionality.
The gnome shell desktop can be separated in two parts: the desktop view (where you see either your desktop, or one or more applications open where you can work on them), and the Activities overview (where you have a panel with several functions and an overview of you active workspaces and open applications organized so that you see each application equally inside a workspace, and where you can only drag them between workspaces and select one as the active one). There's innovation in both these parts, but I believe most of it comes in the Activities overview.
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