There are five different kinds of light used to shade objects in art:
The highlight is the lightest part of a round surface. The highlight is usually inside of the edge that is lit. This gives the surface the look of curving all the way around the back.
It is called a lighted plane if it is a flat surface.
The transitional light describes the middle values between the highlight and the darkest shadow. This is a modeling shadow because it is what makes the object look round. It has soft edges on round objects.
On an angular object there are abrupt changes in value where the planes change direction.
The core of the shadow is the darkest part of the shadow -- the place where the least light hits the object. An object looks rounder when the core of the shadow is in from the edge.
Reflected light is what makes the core of the shadow appear to be away from the edge of the object. It is where light is reflect from a nearby area onto the object. Careful observation of reflected light will help you make objects more convincingly round looking.
The cast shadow is the shadow the object casts onto another surface. This kind of shadow usually has a hard edge, which can help you distinguish it from a modeling shadow. A cast shadow will be darkest closest to the object casting it.