Mutter is an open source project designed to replace the Metacity window manager in the GNOME desktop environment, offering compositing functionality and many other attractive features. The software is distributed along with the GNOME desktop environment, providing decorations for all windows. Technically, it is called a window manager or a window decorator, or both, as it decorates windows and allows users to interact with them.
An important component of a desktop environment
Without a window decorator/manager, any desktop environment would be useless and pretty ugly. The decorator is in charge of many things, for example it allows users to minimize, maximize and close a window, as well as to display the title of the windows. The application uses GTK+ for drawing window frames and it does not expose the concept of "window manager" to the user. In addition. it does not try to be a desktop environment, even though there are some projects that are called window managers but they actually act as a desktop environment, such as Openbox and Fluxbox.
Runs on top of a desktop environment
However, these so called lightweight window managers can indeed run on top of a complete desktop, such as GNOME, Xfce, LXDE or Cinnamon and MATE. Mutter is used by default only on GNOME, but it can be used on any other open source desktop environments. It also acts as a compositing manager, providing off-screen buffer for each window.
It has been ported to Wayland
As of September 2013, Mutter has been successfully ported to the Wayland display server, and it is available for download on Softpedia as a separate package called Mutter Wayland. This will be used by default when GNOME switches to the brand-new Wayland window system.
All in all, Mutter is another important part of the GNOME desktop environment. It allows users to interact with opened windows, it displays a window’s title and makes your desktop experience much more pleasant.