1.A disc which is attached to the axle and rotates as the wheel turns. The discs are also called ?rotors.?
2.Brake pads. Which are housed in a mechanism designed to squeeze them against the rotors.

The calipers and the caliper housings and mounts are rarely mentioned, as they are not subject to the wear and tear of the abuse that is the daily job of the pads and rotors. To stop the vehicle, the pads squeeze the fast rotating discs until the discs stop turning. This is a process of friction, and wears the pads and rotors. The pads are designed to be ?sacrificial? in nature and they are supposed to wear out while not destroying the rotors.

This usually works, up to a point. What happens when you don't worry about your brakes is that the harder bolts which hold the brake pads to the caliper assembly start to stick out above the remaining pad material. These bolts begin to gouge the rotors. The Federal and local Departments of Transportation all have requirements as to how much wear (gouging) can be apparent on the rotor before the rotor surface must be ?turned? (smoothed) by removing more of the material that the rotor is made of. There are also regulations as to how thick the rotor plate must be after being ?turned.?

These regulations are designed to protect the brake system from catastrophic collapse in the case of failure of the rotor. If you have your tires rotated every three to five thousand miles, (with each oil change), the people who rotate your tires will, as part of the tire rotation service, check your brakes for pad and rotor wear. If your car still has drums and brake shoes, they will also check those brakes which are subject to the same punishment each time you decelerate. These people can be considered to be ?experts? and you should pay attention to what they have to say. When your brake pads or shoes are worn down enough that they are eating into the drums or rotors, listen to the ?experts? and replace the pads or shoes and save lots of money in the long run.