According to WikiAnswers:
Consomme is crystal clear while broth will almost always have some suspended materials in it.
Broth is made by simmering vegetables or meat and bones for some time, until the flavor of the simmered food has infused the water. The stock or broth is then usually poured through a strainer to remove most of the particles, bones, skin, and the meat. It's used this way, with no further processing.
Consomme is cleared by adding a step: after sieving as above, egg whites are added to the hot broth or stock. As the egg whites cook, they precipitate into the stock, and begin to bind with some of the particles in the broth. They then rise to the top, appearing as scum or foam. Once the foam cap has formed, pouring off the broth from below, while leaving the cap undisturbed will produce consomme - a broth as clear of suspended particles as water.
at 5:03 PM on Nov. 2, 2008