Brake Pads convert the kinetic energy of the car to thermal energy by friction. Two brake pads are contained in the brake calliper with their friction surfaces facing the rotor. When the brakes are hydraulically applied, the calliper clamps or squeezes the two pads together into the spinning rotor to slow/stop the vehicle. When a brake pad is heated by contact with a rotor, it transfers small amounts of friction material to the disc, turning it dull grey. The brake pad and disc (both now with friction material), then "stick" to each other, providing the friction that stops the vehicle.