National Geographic News has an interesting Video Clip on why dogs may have become “Man’s Best Friend”.
The point of the movie is that dogs have learned to interpret human’s gestures.
I remember hunting with my dad once with his hunting dog, and he was able to direct her to go into a particular clump of brush. I asked him how he did it and he said that she looked back frequently, and he simply held up his right or left hand depending on what direction he wanted her to go.