Shape and Form
Shape and form are two very important visual elements. Every object has both shape and form.
Shape
Shape is how the subject looks in two dimensions. A silhouette, like this photo of tree branches, is an example of shape in a photo.
Form
Form is how the subject looks in three dimensions. Photos are two dimensional, and like painters, photographers have the challenge of depicting three dimensional objects (their subjects) in a two dimensional form (the photo).