Cel shading and soft shading are often used in place of one another, but to fully realize lighting, you have to use both depending on the relationship of the light source to the surfaces at play.
The key here is the light source and what it does when interacting with the surface. Does it penetrate and diffuse into the object? Does it reflect and create glare?
Drawing a bunch of geometrically identical spheres with different materials, like chrome or leather or skin, is a good exercise.
The trouble with shadows is if they're linear like this they make the object seem flat. If they're rounded and follow contours they look more three-dee-alistic, which is totally a word.
The key here is the light source and what it does when interacting with the surface. Does it penetra
