Xgl is a new Xserver architecture layered on top of OpenGL. Xgl can perform intricate graphical operations--such as rendering antialiased fonts--noticeably faster than other available Xservers that do not use OpenGL. The current implementation is Xglx but the future implementation of Xgl will be Xegl.
More important than speed alone, Xgl accelerates complex composite operations, making possible new stunning visual effects through on OpenGL-enhanced composition/window managers like Compiz. These visual effects have broad practical use, making Linux more accessible for the visually impaired, and creating a workspace that is easier to comprehend and navigate for less experienced users.
Xgl must be used in combination with a compositor/window manager to expose all of its capabilities. Compiz is the compositor utility that was developed in conjunction with Xgl.
It is possible that future graphics hardware will support only 3D operations, without any 2D core any more. For this reason, Xgl is an important advance for all operating systems that rely on the X.org graphical foundation.