Within every wheel is a component designed to slow it down on command. In many cases, this is either a disk brake or a drum brake. Both are designed to use friction to slow the spinning speed of the wheels (each brake individually handles one wheel) when the driver presses the brake pedal. In some cases, it can also be used in stability control.
When brakes are used a lot and their effectiveness seems to be reduced every time, this is a condition called "brake fade". This condition can be reduced or even avoided completely by upgrading the brakes to a better design and/or material.
Most modern braking systems have some kind of driver assistance built into the system. The system either has an electrically-powered vacuum pump, or a vacuum line which connects to the intake manifold. With both systems, the vacuum pulls in the same direction as the driver would when pressing the brake pedal. If you want to know how much your vehicle's vacuum assist helps you, push the pedal repeatedly with the engine off, until it gets difficult to push the pedal.
Pictured at the left is a brake fluid reservoir in front of the power brake booster, which has a vacuum assist line leading to the intake manifold (which means this system isn't electric). The brake fluid reservoir is the off-white plastic container, and the brake booster is the large black circle behind it which isn't completely visible. To the left of the reservoir you may see a yellow plastic circle with a black rubber hose leading away from it; this is the vacuum line which leads to the engine's intake manifold.
Brake disks are disks which are mounted to the wheel hub, typically inside the rim of the wheel.
Drilled/Vented Brake Disks
Normal metal brake discs can be improved upon with professionally-drilled brake vents. Some sports cars come with drilled brake vents, and many aftermarket manufacturers produce replacement brakes that are drilled/slotted. Some people may look at them and think these brakes reduce surface area used to create friction, and are only there for aesthetic appeal. This is partially correct, since it does reduce surface area that comes in contact with the brake pad, but the gains from the lower temperatures more than make up for the small loss of friction.
Disk brakes use pairs of pads to clamp down on the disk to create vehicle-stopping friction. Semi-metallic are the most common, but ceramic brakes have better stopping power. Another option is organic pads, but these produce a lot of dust.
In the picture above, the brake pad on the left is new and the friction material is ceramic. The top middle one is a used semi-metallic pad from the same vehicle. The brake pad on the right was a semi-metallic one like the top middle, but it has been worn down to the metal, and should have been replaced earlier. All three brake pads are for a 2002 Ford Taurus.
Some materials are better suited to the heat of heavy braking, but their cost prevents them from being used in most cars. Carbon-ceramic brakes are an example of this. Their high resistance to heat means their surface area can always provide the best braking performance after heavy usage.