http://wiki.secondlife.com/wiki/Sculpted_Prims

From the page:
“A Sculpted Prim is a prim whose shape is determined by a texture. These textures are called Sculpt Textures or Sculpt Maps.

You can use sculpted prims to create more complex, organic shapes that are not currently possible with Second Life’s prim system.”

To create them you’ll need a 3D program like Blender:
http://www.blender.org/