A brake rotor is a metal disc, attached to a vehicle’s wheel hub, that receives friction from the brake pads to stop the vehicle. The rotor is situated within a caliper, which houses brake pads situated on either side of the rotor. When you press your brake pedal, hydraulic pressure from the brake fluid pushes a piston(s) causing the brake pads to squeeze the rotor and bring your wheels to a stop. Brakes consisting of a rotor, brake pads, and calipers are called disc brakes.
The other common brake type is the drum brake. These have shoes instead of pads, and drums instead of rotors. Most new cars today have either disc brakes on all four wheels or disc brakes on the front wheels and drum brakes on the rear wheels.
Brake rotors wear down over time due to contact with brake pads. But you don’t necessarily need new rotors with each brake pad replacement. Most rotors can be resurfaced one or more times to extend their life. But each resurfacing makes the rotors thinner, and sooner or later you’ll need new ones. Always follow the recommendations of the vehicle manufacturer when replacing rotors.