Quad is a very efficient basic 1x1 geometry used mainly for holding textures. It only renders a basic set of vertices and texture coordinates, no normals, binormals, tangents. Its main purpose is to hold basic images with Phong Materials. By default, it renders a 1x1 pixel surface and need to be scaled up correctly.
Note: This plug-in is located in: Plugins -> Geom plug-ins -> Default