Basically a polygon and nurbs hybrid. It is helpful for converting between polygons and nurbs.. I thought subdivs were only available in unlimited versions of maya, but it seems ple6.0 has them.

Maya definition of "subdivision surface": A hybrid surface type that provides both the smooth organic forms of NURBS modeling, and the detail and extrusion capabilities of polygonal modeling. A subdivision surface can have different levels of detail (density of control points) in different regions.

Subdivision surfaces allow you to model complex shapes from a single surface. They can have varying levels of detail (see hierarchy).

Also known as subd surface.