Shape
Shapes are often categorized as  planes or volumes. Planar shapes usually appear flat and decorative. But planes can take on the illusion of dimensionality by adding spatial clues such as shading and perspective. These shapes, now called volumes, appear to take up a certain amount of space.  
It is often helpful to think of a complex shape as a combination of simpler shapes.  If a complex form is to be depicted three-dimensionally, then imagining the form as a group of simple solids can often help render it convincingly.