Learning the rules that light and shadow obey, will help you understand what you are seeing and make it easier to draw!
Highlight: the area facing the light source and receiving the most light.
Halftone: the transition between your highlight and shadow.
Core Shadow: the darkest area of an object.
Reflected Light: light from the source bounces off an object and creates a light area within the shadow, but it is never bright as the highlight.
Cast Shadow: the silhouette of an object projected opposite the light source.
*Don't forget: reflected light is never as bright as your lighted area.*