The power brake booster assists the driver in applying the brakes by aiding the movement of the master cylinder, applying a small force in the same way as the brake pedal.
Most internal combustion engine vehicles have a vacuum-based power brake booster, since the vacuum pressure is generated by the engine naturally. A vacuum brake booster is usually easy to spot, being a large (usually painted black) cylinder shape behind the brake master cylinder.
Hydraulic Brake Boosters
Hydraulic assist, known as "Hydroboost" on GM vehicles, uses hydraulic pressure that is either on its own system or part of the power steering system. This method isn't used as often, and when it is shared with the power steering, a pump failure or pressure leak can cause two systems to fail at once.