Here is a more advanced lesson in shading. Using the example of the pear, you can see how the simple shading techniques have been used. The light source comes in from the left and creates a cast shadow on the right side of the object. The object is lightest where the light source hits it and darkest where the light is shaded the most.
The simple elements are there, but now there are a few more elements to deal with.
Highlight: Where the light source hits the object
Core Shadow: The largest area or shade
Mid-Tones: The transitional area between the lightest and darkest areas
Reflected Light: The subtle highlight from indirect light
Cast Shadow: The shadow the object casts
Highlight: Where the light source hits the object
Core Shadow: The largest area or shade
Mid-Tones: The transitional area between the lightest and darkest areas
Reflected Light: The subtle highlight from indirect light
Cast Shadow: The shadow the object casts