What are disc brakes? What other types of brakes are there?

The disc brake is a device for slowing or stopping the rotation of a wheel. A braking disc, usually of steel, is rigidly connected to the wheel. To stop the wheel, the braking pads are forced mechanically or hydraulically against the disc on both sides. Friction causes the disc and wheel to slow or stop.

The design of the disc varies somewhat. Some are simply solid steel, but others are hollowed out with fins joining together the disc's two contact surfaces. This "ventilated" disc design helps to dissipate the generated heat. Many motorcycle brakes instead have many small holes drilled through them for the same purpose.

Experiments with disc-style brakes began in the 1890s, but the first designs resembling modern disc brakes began to appear in Britain in the late 1940s and early 1950s. They offered much greater stopping performance than comparable drum brakes, including much greater resistance to "brake fade" (caused by the overheating of brake components), and were unaffected by immersion (drum brakes were ineffective for some times after a water crossing, an important factor in off-road vehicles). Not to mention, discs are more reliable then drum brakes due to the simplicity of their mechanics and the lack of parts compared to the drum. They have now become standard in most passenger vehicles (though some retain the use of drums on rear brakes to keep costs down).

Cars, motorcycles, and some bicycles use disc brakes.

Other types of brakes/systems (I'm sure there are more, but this should answer the question sufficiently)

REGENERATIVE braking is any technology that allows a vehicle to recapture and store part of the kinetic energy that would ordinarily be lost when braking. A simpler technology that does not recover energy but uses similar principles is known as dynamic braking. Both are most commonly seen on electric or hybrid vehicles. Braking is accomplished by electrically switching motors to act as generators that convert motion into electricity instead of electricity into motion. Traditional friction based brakes must also be used to provide powerful response in rapid situations.

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.

In the first drum brakes in cars the shoes were mechanically operated with levers and cables.

The shoes in drum brakes are subject to wear and the brakes needed to be adjusted regularly. In the 1960s and 1970s brake drums on the front wheels of cars were gradually replaced with disc brakes.

Another type of drum brake is where a friction belt is wrapped around the outside of the drum and tightened. This type was used for the parking brake on the central drive shaft.This type of band brake is also used in automatic transmissions and aerobic exercise cycling equipment.

AIR brakes. In aeronautics air brakes or "spoilers" are a type of flight control used on aircraft to reduce speed during landing. On railways an air brake is a brake operated by condensed air. On trains, this replaced the outdated VACUUM brake.

VACUUM brake a type of braking system used on trains. Vacuum braking is for all practical purposes now a dead technology; it is not in large-scale use anywhere in the world, supplanted in the main by air brakes.

Vacuum brakes permit the automatic application of brakes down the length of a train from a simple control in the driver's hand. They are also failsafe, since they default to an applied state; power in the form of vacuum is used to release the brakes, so if vacuum is lost due to malfunction or the train breaking apart, the brakes are automatically applied.

ANTI-LOCK brakes (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: to allow the driver to maintain steering control and to shorten braking distances.

The antilock brake controller is also known as the CAB (controller antilock brake).

A typical ABS is composed of a central electronic unit, four speed sensors (one for each wheel), and two or more hydraulic valves on the brake circuit. 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), moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel.