A disc brake consists of a metal disc attached to the wheel hub that rotates with the wheel. Callipers are attached to the frame or fork along with pads that squeeze together on the disc. A bicycle disc brake may be mechanically actuated, as with a Bowden cable, or hydraulically actuated, or a combination of the two.
Disc brakes are used mainly on mountain bikes ridden off-road, but sometimes on Road Bikes, hybrid bicycles and touring bicycles. A disc brake can also be employed as a drag brake.
Disc brakes tend to perform equally well in all conditions including water, mud, and snow due to several factors: 1) The braking surface is farther from the ground and possible contaminants like mud which can coat or freeze on the rim and pads. 2) Disc brake pads when fully retracted ride much closer to the braking surface than rim brake pads.This better prevents a buildup of water or debris under the pad. 3) There are holes in the rotor, providing a path for water and debris to get out from under the pads.
4) Wheel rims tend to be made of lightweight metal. Brake discs and pads are harder and can accept higher maximum loads. Unlike most rim brake designs, disc brakes are compatible with bicycle suspensions.