Lambert shading:

Lambert shading is depending upon calculating a single normal vector for the surface (which is then compared to the lighting vector and the viewpoint vector to find out the colour). Gouraud shading is based upon calculating a vertex normal rather than a surface normal. A vertex normal is an artificial construct (a true normal may not exist for a point such as a vertex). A vertex normal may be thought of as the average of the normal of all the polygons that share that vertex.