Mutter is the window manager

Window manager

A window manager is system software that controls the placement and appearance of windows within a windowing system in a graphical user interface. Most window managers are designed to help provide a desktop environment...

 for which GNOME Shell is a plugin in GNOME

GNOME

GNOME is a desktop environment and graphical user interface that runs on top of a computer operating system. It is composed entirely of free and open source software...

 3, replacing Metacity

Metacity

Metacity was the window manager used by default in the GNOME desktop environment until GNOME 3, where it was replaced by Mutter. The development of Metacity was started by Havoc Pennington and it is released under the GNU General Public License....

. (The name "Mutter" is short for "Metacity Clutter" Mutter uses a graphics library called Clutter, and also supports OpenGL

OpenGL

OpenGL is a standard specification defining a cross-language, cross-platform API for writing applications that produce 2D and 3D computer graphics. The interface consists of over 250 different function calls which can be used to draw complex three-dimensional scenes from simple primitives. OpenGL...

 by Clutter.)

The Mutter window manager can function as standalone window manager application for GNOME-like desktops, and serves as the primary window manager for the GNOME Shell desktop, which is an integral part of the GNOME 3.0 release. Mutter is extensible with plugins and supports numerous visual effects.

Performance

Games run with a performance hit when using a compositing window manager

Compositing window manager

A compositing window manager is a type of window manager. A window manager is software that draws a graphical user interface on a computer display – it positions windows, draws additional elements on windows , and controls how windows interact with each other, and with the rest of the desktop...

; in June 2010 Phoronix

Phoronix

Phoronix is a technology website that offers product reviews, Linux distribution screenshots, interviews, and news while maintaining a pure Linux orientation. Phoronix was started in June 2004 by Michael Larabel, who currently serves as the owner and editor-in-chief. The name Phoronix is a...

 evaluated this as similar for Mutter and Compiz

Compiz

Compiz is one of the first compositing window managers for the X Window System that uses 3D graphics hardware to create fast compositing desktop effects for window management. The effects, such as a minimization effect and a cube workspace are implemented as loadable plugins...

.