A brake rotor, also called a brake disc, is the circular part that a car's brake pads grab onto when the brake pedal is pushed. Brake rotors help create the friction needed to stop a car's wheels from turning. They come in two varieties: drilled and slotted.
Drilled brake rotors have tiny holes drilled in a wave-style pattern along the surface of the disk. These holes help cool the brakes, which get hot because of the friction, by allowing the heat to escape. A brake rotor with a smooth surface does not allow the brakes to cool down, which potentially causes brake fade. This makes it more difficult to stop the vehicle, and in extreme cases, such as under heavy loads, excessive heat can lead to brake failure.
Slotted brake rotors work on the same principle, but instead of holes going through the disc, slots are carved out at intervals to provide the heat escape. These are more often found on race cars because they stand up to hard, fast driving better than the drilled rotors. Slotted brake rotors are harder on brake pads, which must be replaced more frequently, and are rarely installed on performance cars that are destined for street use only.