Drum Brake
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.
A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against the inner surface of a rotating drum. The drum is connected to a rotating wheel.
Disc Brake
With disc brakes, a disc attached to the wheel hub maybe 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.
Antilock Braking System (ABS)
An anti-lock braking system (commonly known as ABS, from the German name "Antiblockiersystem" given to it by its inventors at Bosch) is a system on motor vehicles which prevents the wheels from locking while braking. The purpose of this is twofold:
A typical ABS is composed of:
The electronic unit constantly monitors the rotation speed of each wheel. When it senses that one or more wheel is rotating slower than the others (a condition that will bring it to lock) it moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel.
Source: CDX Global
