Drum brakes have a drum attached to the wheel hub, and braking occurs by means of brake shoes, expanding against the inside of the drum.
With disc brakes, a disc attached to the wheel hub is clamped between 2 brake pads.
On light vehicles, both of these systems are hydraulically operated. The brake pedal operates a master cylinder. Hydraulic lines and hoses connect the master cylinder to brake cylinders at the wheels.
Most modern light vehicles have either disc brakes on the front wheels and drum brakes on the rear, or, disc brakes on all 4 wheels.
Disc brakes require greater forces to operate them. A brake booster assists the driver by increasing the force applied to the master cylinder, when the brake is operated.
The antilock braking system prevents wheel-lock or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced.
It consists of a brake pedal, a master cylinder, wheel speed sensors, the electronic control unit or ECU, and the hydraulic control unit, also called a hydraulic modulator.