The vacuum booster is a metal canister that contains a clever valve and a diaphragm. A rod going through the center of the canister connects to the master cylinder's piston on one side and to the pedal linkage on the other.
Another key part of the power brakes is the check valve.
The photo above shows the check valve, which is a one-way valve that only allows air to be sucked out of the vacuum booster. If the engine is turned off, or if a leak forms in a vacuum hose, the check valve makes sure that air does not enter the vacuum booster. This is important because the vacuum booster has to be able to provide enough boost for a driver to make several stops in the event that the engine stops running -- you certainly don't want to lose brake function if you run out of gas on the highway. In the next section, we'll see how the booster works (and check out a cool animation!).