Good drawing involves awareness of the lighting of a subject. The play of light and shadow can drastically affect the appearance of a subject. The placement and intensity of the light source affects the lights, darks and shadows of the subject. Understanding the light in relation to the subject will influence the depth and mood of your drawing. The aspects of lighting include highlights, form and cast shadows, and reflected light.
The light source is the origin of light that influences the lights, darks and shadows of a scene. Most light sources (whether natural or artificial) are above a subject. A scene may have more than one light source.
A highlight is a bright spot where light reflects off an object.
A form shadow appears on an object, displaying the form of the object.
Typically darker than form shadows, a cast shadow is caused by an object casting a shadow onto another surface.
Reflected light is light that is bounced from one surface onto another. Typically, reflected light is not as bright as the primary light source. It may throw subtle light onto an area that would have otherwise been dark.
The lights, darks and shadows of a scene are influenced by the light source.
Facial light effects can be observed with a plaster cast face or a Styrofoam mannequin head that you can find at art or hobby stores.