There are, quite simply, only two kinds of shadows.. Form shadows and cast shadows.
Form shadows are found on the opposite side of any lighted surface. Form shadows are lighter in value as a whole and softer than cast shadows. But they do vary. Depending on the object size, shape, surface quality and distance from the source of the light, the point where light meets shadow will determine whether it is a soft or hard edge. A hard edge will occur on a cube while a soft edge occurs on a ball.
A cast shadows is what we generally think of as shadow. It is when an object blocks the light and casts a shadow, such as a tree in the light and the shadow on the ground. The cast shadow is a darker shadow, because the light has been blocked. It is darkest where the shadow starts next to the object. The farther the shadow is from the object the softer and lighter the shadow becomes. Thus, a shadow isn’t the same throughout.
Remember to squint when you are painting shadows to see this differences more clearly.