Vertex Normals vs. Surface Normals
We all know what a surface normal is (if not, it’s the normal to the plane that contains thesurface). So how can a vertex (i.e., a point), have a normal? Strictly speaking, it can’t.What vertex normals provide is a means of simulating smoother surfaces during lightingcalculations when using procedures such as Phong or Gouraud shading (sometimes justcalled Smooth shading). Imagine a polygonal mesh of a human: technically, this mesh isjust a bunch of flat polygons. But really, this mesh is simulating the smooth surface of ahuman body. If all pixels within a polygon were lit identically, the “flatness” of eachpolygon would be starkly obvious; but by using vertex normals, we can light each vertexon a triangle differently, thus causing a smoother appearance. The trick is to generatethese vertex normals so that they actually enhance this smooth appearance.
http://www.emeyex.com/site/tuts/VertexNormals.pdf