The primary purpose of an automobile is to travel from place to place. However, you need (at minimum) some way to stop when you get there, and it is often convenient to have some way to stop while you are on the way there. This is where the brake system comes in.
Modern vehicles basically use two types of brake, both hydraulically activated. By far, the more superior version is the disc brake, which offers greater stopping power and resistance to heat, especially when using a ventilated brake rotor. This type uses a pair of pads which grip a brake rotor (or disc.)
The common alternative is the drum brake, which presses brake shoes against the inside of a brake drum. It is heavier, retains more heat, and has more moving parts, yet automakers continue to install them on the rears of vehicles, ostensibly because they are more resistant to splashed water, oil, and other contaminants.
The most recent development in braking is the antilock braking system or "ABS". A simple computer receives signals from the wheels which indicate the speed at which they are rotate and a hydraulic pump system stops braking at any wheel which is moving significantly slower than the other wheels. This prevents the wheels from locking up and producing a skid. Depending on the age of the ABS system, it may or may not be able to stop slower than a talented human driver, but the most important aspect of ABS is that it allows you to stomp on the brakes as hard as you like and still steer the car, which you cannot do when they are locked up and you are skidding.