Brake rotors are the parts of the wheel which is squeezed by the brake pads around the edge of the brake drum.
This enables the brake pads to apply friction to a part of the wheel which can then cause it to slow and eventually to completely stop. Depending on the specific vehicle, the brake rotors can be made from a variety of different materials and in a variety of different designs. The majority of brake rotors are made from a heavy cast iron, while some others are made from Kevlar, carbon or other specialised materials.
Some brake rotors are 'cross-drilled' and this means that they have several small holes running through them. This was original done in racing vehicles because gas would otherwise sometimes become trapped between the brake rotors and brake pads and this would prevent them from being as effective. Modern brake pads however do not present this problem however so newer racing vehicles and cars will not feature cross drilling.
Instead cross drilling is now more common on motorbikes and high performance mountain bikes. As well as preventing the build-up of trapped air, cross drilled brake rotors also aid in the dissipation of heat.
This is important as otherwise it could otherwise warp the rotor when the brakes are applied quickly (a lot of heat is generated from the friction).
Other brake rotors are slotted, and this means that they have shallow grooves carved into them which have a similar effect to cross drilling in sports cars allowing air to escape. However slotted brake rotors are not practical for many cars as they mean the surface is less smooth and this then means that the brake pads will wear down more quickly.
Though steel and iron are the most common materials for brake rotors, some vehicles will use reinforced carbon which shows higher performance at greater temperatures and is much lighter making it very suited to racing and sports cars.
Less common than carbon rotors but still used quite regularly are ceramic materials. Here the main advantage is again the light weight, but they also have relatively minimal maintenance requirements.