A brake pad is the friction material or lining which is secured to metal plates. They press against the brake disc or rotor to enable the wheel to stop. They are to be distinguished from brake shoes which press against the inside of a drum. It is also called "brake block". Sometimes the term brake pad refers to both the pad and the metal backing.
The most common problem with brake pads is that they wear over time. When they are pressed against the brake disc or rotor, the friction causes the brake pad material to deteriorate. If the brake pad material wears completely off, the metal backing can wear grooves into the rotors or disc causing them to become damaged.
Brake pads should be replaced according the the manufacturer's specifications. While brake pad replacement isn't covered under a manufacturers' factory warranty or an extended auto warranty, there are maintenance plans available that will cover the cost of any scheduled maintenance.