All Mountain Bike Parts Descriptions

#12) Wheel Hubs - The center of the bicycles wheel.
  Hubs complete the rims, spokes, and tire-tube combination to become, at last, a wheel. Front hubs are very basic; an axle, cones, and hub shell to attach the wheel to the fork.
  Rear hubs however, could have internal gears, coaster brakes, or both. There are also cassette body's, which are built on to multi geared cogs or just plain old threads to spin on freewheels. I've seen drum brakes, electric generators (which were very low drag for lights) built into hubs, and now disc brakes using six or eight inch rotors.

Bike Parts Diagram