How Light falling on a subject affects shape and contour

 How does the direction light is coming from affect the subject? Here are some late afternoon shots taken at the local fruit stand which illustrate how shape and definition change depending on where the light is falling.

Front Lighting
Late Afternoon February 2013
Front lighting gives good detail on the surface of the oranges.
Oranges would appear somewhat flat if it were not for the
low angle of the sun and the stacking

Side Lighting
Late Afternoon February 2013
Low angle sun coming from the left makes
distinct highlight and shadow sides of the oranges.
Oranges appear very round.

Back Lighting
Late Afternoon February 2013
Camera is pointed almost directly towards the sun.
The majority of each orange looks dark.
The rims of the curved tops of the oranges are highlighted.
Some detail shows in the shadows because light has
bounced off the pile of oranges, illuminating the back side.