Shading is showing the objects' roundness or shape; shade is the blocking of a light source; and shadow is the variation between black and white with the darkest part at the center and the 'shadow' fades at the edges.
Chiaroscuro is Italian for<em> light and shadow</em>
Painters use this technique when they need to <u>create the illusion of light</u> on objects and people
I've attached a pic for example :D