The brake rotor is the American term for the brake disc and is a smooth circular steel plate that spins with the road wheels and is used with the brakes to slow the vehicle.
Located behind each wheel the brake rotor is straddled by the brake calipers which contain the brake pad system.
The rotor is in a fixed rotation with each wheel, when the brakes are applied, the brake pads, on each side of the rotor, which are located in the caliper, are forced hydraulically onto the rotor which, through friction, slow the rotation of the disc and subsequently the wheels and the vehicle.
Problems with the rotor can include excessive scoring of the smooth surface, due to the failure to renew brake pads, or corrosion. Removal of the rotor requires caliper removal.
Due to the high energy forces, heat, friction and stresses involved with the braking system in the area of the rotors and wheels, it's of the utmost importance that all components are in good order. The brake rotor is specifically manufactured for the make and model of your vehicle.