These are components of braking systems using brake discs and are steel backing plates with friction material bound to the surface. They make contact with the brake disc in order to stop wheel rotation. Brake pads should be replaced regularly and are usually equipped with a thin piece of soft metal that causes the brakes to squeal when the pads are too thin.
It is composed of soft but extremely tough and heat-resistant material which is attached to the brake pad or brake shoe (usually using high temperature adhesives or rivets). The brake lining in either case is the part which actually makes contact with the brake disc or drum.