Forms are shapes that are usually three-dimensional. However, in a painting, shapes and objects can be highlighted and shaded to give the visual illusion that something is three-dimensional.
The Louisa Villea is a two-dimensional oil painting. This means it is a picture on a flat panel or canvas. The objects in the painting can be measured two ways, height and width (up and down). Highlighting and shading are used to create the illusion that objects in the painting have a third dimension or depth.
Proper highlighting and shading give shapes the appearance that they are realistic and have form.
Notice the subtle highlights and soft shadows on Louisa and the cherub. The slight variation in color and lighting turns a flat shape into a realistic looking form.