What is the difference between disc brakes and drum brakes?

Disc brakes are generally considered superior to drum brakes for several reasons. First, they dissipate heat better (brakes work by converting motion energy to heat energy). Under severe usage, such as repeated hard stops or riding the brakes down a long incline, disc brakes take longer to lose effectiveness (a condition known as brake fade). Disc brakes also perform better in wet weather, because centrifugal force tends to fling water off the brake disc and keep it dry, whereas drum brakes will collect some water on the inside surface where the brake shoes contact the drums. -----quoted from cars.about.com