Warning! This tutorial is not intended to cover each of the shader's properties. That's what the original product documentation is for. It only intends to facilitate, as far as possible, its understanding. Bear in mind, that it's also a personal way of seeing it. Thanks to my wife (and Google's translator), for helping me with my lousy English!
Face / Polygon: area delimited by edges and vertices. A face is coplanar, when all its edges and vertices are on the same plane. We denominate normal to the vector that is perpendicular to a coplanar face.