Elements of Photography:
Three-dimensional representation of objects, usually through the use of lighting and shadows.
Form is a three dimensional representation of an object. A third dimension (thickness) to the Shape yields Form.
Photography (and art) is a two dimensional form lacking the depth which poses a challenge to you as a photographer to somehow represent the third dimension by creating an illusion of depth.
By using light and shadow you can create an illusion of depth in your photograph.