We created skin as a method for sculpting free-form surfaces. The idea is that it is not always intuitive for a modeler to think in terms of the actual surface of a shape. Instead, we would like the modeler to be able to focus on the underlying structure.
Skin is a triangulated particle system.
That is, each vertex in a triangle mesh is a particle. The particles move under the influences of their neighbors, as well as a collection of underlying shapes called skeletons. We can thus scult very complicated surface by creating an underlying skeleton and growing skin over it. Here is a face model made by growing skin over a skeleton.