Brake pads are originally made from organic materials such as carbon and asbestos which was held in place by a resin. The use of asbestos however was banned in most territories though some non-metallic brake pads are still sold (though only vehicles that are designed for them can use them). Recently Kevlar (the material used in bullet proof vests) is more commonly used and most brake pads today are semi metallic though the exact formulas are often secret. They will often however use copper, steel wool shavings, brass and more and are again held together by a resin.
As brake pads are mostly metallic they can now last over thousands of miles though they are noisier than their organic predecessors. This noise is caused by the shavings rubbing against the steel rotors every time they go around. Some brake shoes however are marketed as being quieter than others.