When you press the brake pedal to slow your car to a stop, a series of mechanisms are put to work. Most modern cars are equipped with disc brakes, which involve calipers applying pressure to brake rotors. The rotors are affixed to the wheels, so when the calipers slow them down, the wheels stop spinning as well. The calipers' grip on the rotors is heavily dependent on friction, and this could dramatically wear down the rotors if there was not a protective barrier between the two parts. This is where the brake pads come in. The calipers press them into the rotors and their material creates enough friction that it slows the rotors.
Brake pads can be made of organic materials, ceramic or even metal. They used to be made almost exclusively of asbestos, since the material was effective at absorbing and dissipating friction-generated heat. Since it turns out asbestos is extremely dangerous to people, new brake pads were developed to take the material's place. Organic options can be made out of glass, rubber or resins that can handle high temperatures. They tend to be softer and quieter than pads made from ceramic and metal, but they will also wear out faster. This also renders them a poor choice for larger vehicles and high-performance cars.
On the opposite end of the spectrum are high-performance ceramic brake pads. These are made of ceramic fibers, filler material and even copper fibers in some cases. They are so effective in dissipating heat that you can find them on many race cars. Plus, they're lightweight, which will help to preserve a vehicle's performance, and they take longer to wear out than other brake pads. These are great for sports cars that spend time on the track as well as the road. However, they tend to cost extra and aren't typically the best choice for the average driver.
Metallic or semi-metallic brake pads are the most commonly-used, since they offer durability as well as decent performance. These brake pads are made of a mixture of different metals, which may include iron, steel, graphite and copper. Metal weighs more than other brake pad materials, which could have an adverse effect on your vehicle's fuel economy. It's also harder and stronger than ceramics and organic substances, allowing you to drive further before needing to replace them.
Every time you brake, the pads will wear down a little further, and eventually they will need to be replaced. Most brake pads include a small piece of metal that becomes exposed once they are worn to a certain point. This causes a high pitched squeal when you use the brakes, acting as an alert for drivers that replacements will be needed soon. The rule of thumb is to replace the brake pads when the thickness reaches between a quarter- to a half-inch. In some vehicles, the pad thickness can be checked easily by looking through the wheel, but others may require a part or two to be removed first.
If you wait too long to replace the brake pads, you risk damaging the rotor. If this part warps, your brakes may not be as effective. While changing the pads, use the opportunity to inspect the rotors for scoring or uneven surfaces. If you find evidence the rotors are damaged, they may need to be replaced as well.