Form describes volume and mass, or the three-dimensional aspects of objects that take up space.
Forms can and should be viewed from many angles.
When you hold a baseball, shoe, or small sculpture, you are aware of their curves, angles, indentations, extensions, and edges - their forms.
Form in Fine Art
Space can be felt in de Zurbaran's painting because of the overlapping of forms (two of the vessels and the saucers they sit on, and the table under all of them.) Shading creates three-dimensional forms out of shapes.
Source: Elements and Principles of Design: Student Guide with Activities, published by Crystal Productions