Typical replacement rotors are made of grey cast iron. Most of them are produced in China and Brazil, and they range in quality from SAE grade G1800 to G4500. Brake rotors are a simple device, a solid piece of metal it is there to provide a friction point of contact between the pads. The pads are pressed against either side of the rotor by the calliper. As the pad and rotors work together to stop your vehicle repeatedly, the pads and the rotors are worn down. In the past when parts were more expensive and the labour was cheaper it was common to get a rotor turned when replacing the pads. This was so that the flat surface of the new pad would be met with a flat surface on a rotor. Now it is typically cheaper to just buy new rotors. The problem with only changing your pads and not the rotors is because of the groves that were developed by the old pads. The pad material rarely wears completely evenly, and this causes groves to form on the rotor, which are fine because the groves in the rotor match with the groves in the pad that created them, but not a brand new pad. Putting a new pad on a grooved rotor will not allow complete surface contact to occur, so stopping power is reduced, tendency to squeal or make noise is increased and the pad wear is increased.
Rotors that are for vehicles that see typical daily use are flat faced, and under normal driving conditions a flat faced rotor is the best rotor to use. Slotted and drilled rotors are typically for people that are occasionally tracking their vehicles or wish to show off a fancier rotor when at a car meet. They offer no advantage in terms of braking under normal driving conditions. A cross drilled rotor may provide additional cooling under severe and repeated braking conditions, but it also has more tendency to crack and will wear your pads a bit faster.