Form as an element of visual design is slightly different than shape. A shape is mostly 2 dimensional without any depth whereas form is 3 dimensional and has depth and shadows to it. When your light source comes from a side of your subject that causes a shadow to appear on its opposite side, you are showing the the subject has depth or is 3 dimensional. An example of this would be a beach ball photographed with side light. The ball would cast a shadow on the beach and on the side that is not lit which makes it 3 dimensional on the image.
This illusion can be made from any subject that has enough mass to create a shadow. Eggs can create a shadow on white paper even though they are white, as long as the light is from the side. If the light was directly overhead, it would not cause a shadow to appear and would render the egg to appear 2 dimensional. The image below is another example of an image using form as its visual design.
If the light source was directly behind your subject, it would cast the shadow in the front but would still make your subject appear 2D because it would create a silhouette. If the light is directly in front of your subject, the shadow would appear behind your subject and cast no shadow on the subject causing the image to appear flat or 2D.
So the difference between shape and form are the shadows and where they land in relation to the subject.
Our next post in this series will be on lines as a design element so follow our blog so you can continue reading this series as they get posted.