Difference between master cylinder and wheel cylinder?

The master cylinder is under the bonnet (hood) attached to the brake servo and is where you top up the brake fluid, sometimes it also serves for the clutch fluid in a car with a hydraulic clutch. Pipes come from this to the individual brake calipers on each wheel, sometimes via an ABS ( Anti-lock Braking System) unit.

A wheel cylinder is at the other end of one of the above mentioned pipes on a car with drum brakes and brake shoes. The wheel cylinder is between the two shoes at one end, and forces them open to contact with the drum to stop the vehicle when the brake pedal is pressed.