GNOME 3 has two interfaces:
- GNOME Shell is the new standard layout using the Mutter window manager. It acts as a composite manager for the desktop, employing hardware graphics acceleration to provide effects aimed at reducing screen clutter.
- fallback mode is similar to GNOME 2, it uses gnome-panel/Metacity instead of gnome-shell/Mutter. No hardware acceleration is required to run fallback mode.
GNOME-session automatically detects if your computer is incapable of running GNOME Shell and starts fallback mode when appropriate. When you are on fallback mode you can still replace GNOME's default window manager with your preferred one.