Most automotive systems in use today utilize front disk brakes and rear drum brakes, but 4 wheel disk systems are also fairly common. With disk brakes, the rotor spins with the wheel and the pads move out and rub the rotor when the brakes are applied. Most disk brakes use floating calipers, the caliper slides in and out as the brakes are applied and released. The piston moves the inside pad out and pushes the outside pad into the rotor by sliding the caliper back toward the rotor. With a drum brake assembly, the drum, shaped like a pie plate, rotates with the wheel and the fixed shoes move out to engage the drum when the brakes are applied.
Most systems use a booster that uses engine vacuum to increase the force applied to the brake pedal.
Disk brakes require higher application pressure than do drums because they are not self actuating. When drum brakes are applies the lead shoe helps bring the trailing shoe into the drum. ABS Antilock brake systems use a computer to monitor and control the braking system. Essentially the computer monitors the rotational speeds of the wheels and then releases the brakes when the wheels are about to lock up. The braking system including the tires is most effective, i.e., produces the most retardation, before the wheels lock up and start to slide.