GNOME Shell provides core user interface functions for the GNOME 3 desktop, 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 visually attractive and easy to use experience.
Mutter is a window and compositing manager that displays and manages your desktop via OpenGL. Mutter combines a sophisticated display engine using the Clutter toolkit with solid window-management logic inherited from the Metacity window manager. While Mutter can be used stand-alone, it is primarily intended to be used as the display core of a larger system such as GNOME Shell. For this reason, Mutter is very extensible via plugins, which are used both to add fancy visual effects and to rework the window management behaviors to meet the needs of the environment.