Brakes with a disc called a rotor near the hub and calipers that squeeze pairs of pads against that disc.
A disc brake is a type of brake that uses the calipers to squeeze pairs of pads against a disc called a "rotor" to create friction. This action slows the rotation of a bicycle wheel, either to reduce its rotational speed or to hold it stationary. The energy of motion is converted into waste heat which must be dispersed.
There are two common methods used to actuate disc brakes on a bicycle: hydraulically or mechanically.
Hydraulically actuated disc brakes utilize hydraulic lines between the brake levers and the calipers. Mechanically actuated disc brakes use the more traditional brake cable to transfer force from the brake levers to the caliper. There are rare instances used in special configurations of a hybrid of the two actuations that use a mechanical brake cable to transfer force to the a hydraulic brake assembly located at the caliper area.