Most shading is done based on the vertices and the normal of the poly in relation to the light source. This means that a light towards the vertex of a very big poly will seem to diffuse out, as it effects the other vertices less. That is vertex sahding. The problem with this, is that if you have a light in the middle of a poly the entire poly looks evenly affected by the poly, even if it isn''t; per pixel shading calculates each pixel''s shading based on light individually.