A disc brake uses a set of calipers to press braking pads against a disc attached to the wheel. The friction from this contact slows the wheel and, by extension, slows the vehicle. Most cars have disc brakes attached to the front wheels and older drum brakes on the rear, while some have disc brakes on all four wheels for maximum control.
Disc brakes offer a number of advantages over drum brakes. The centrifugal force of the spinning brake disc shakes off water, while a drum brake allows water to collect inside the cylinder attached to the wheel. Disc brakes also take longer to wear out, since the calipers and brake pads apply force on both sides of the disc rather than grinding against the inside of a brake drum. Cars typically use drums and discs simultaneously, however, since drum brakes are cheaper and can serve as a parking brake.
Disc brakes can fail in a variety of ways. If the pads wear down, the calipers can scar the disc, damaging its surface. Excessive braking heat can cause warping or cracking of the brake disc. These forms of damage can cause inconsistent braking and make the car wobble or jerk while the brakes are applied. In addition, a car left parked for a long period of time may build up rust on the disc, further impeding braking ability.