Chris McFarlen created a patch for Blender to enable weighted creases for subdivision surfaces. This allows for making sharp edges with subsurfs. Ton Roosendaal has adapted the patch further by introducing edges for the mesh datastructure, which as a bonus also speeds up mesh drawing in wire mode.

Creases are a property of mesh edges, and can be edited in mesh Edit Mode when the mesh is a SubSurf. Select the edges you want to have creased, and use Mesh -> Edges -> Crease Subsurf (Shift E). Move the towards and away from the edges to change the amount of sharpness.

You can view your edges by enabling "Draw Creases". The color of the edges depends on the sharpness (weigth factor) of the edges. Black means that no weight has been applied, Edge select color to the fullest means full weight has been applied to the edge. The color of edge select comes from Themes.

Different crease sharpness settings, images by Chris McFarlen.