When the brake pedal is applied, it acts as a lever to compress a hydraulic cylinder called the "Master Cylinder". This tranfers the mechanical pressure of the brake pedal into hydraulic pressure that is used to stop the car. This pressure is then distributed to all of the brakes of the car. Some systems use a "brake booster" which uses the vacuum generated by the engine to boost the power that is applied to the brake pedal. This makes stopping much easier.