Brake rotors take enormous heat and force every time you stop, so the material they are made of matters a great deal. Most are one material, with exotic options at the high end. Here is what brake rotors are made of.
Cast iron is the standard
The vast majority of brake rotors are made of gray cast iron. It is cheap, absorbs and sheds heat well, resists wear, and provides good friction with brake pads - see how disc brakes work. Its main downsides are weight and a tendency to rust on the surface.
Coatings and alloys
Many quality rotors add a zinc or geomet coating to fight rust on the non-friction areas - see coated vs uncoated rotors. Some performance rotors use higher-carbon iron or specific alloys to better resist heat cracking; see rotor heat cracks.
Carbon-ceramic at the top
Exotic and high-performance cars may use carbon-ceramic rotors, which are far lighter, handle extreme heat, and resist fade - see carbon-ceramic brakes. They are very expensive and usually reserved for supercars and track use.
Which is right for you
For nearly all street vehicles, a quality coated cast-iron rotor - drilled, slotted, or plain - is the right choice; see drilled vs slotted. Pair it with good pads and fresh hardware.