I believe the reference is that slotted or cross-drilled rotors are supposed to allow air to flow through the rotors and cool them more effectively. I'm not so sure that's the point, though. I thought the reason for slotting and/or cross-drilling was to allow the brake dust and gases that build up between the pads and rotors to escape, thus keeping better contact between them and providing better performance.

Slotted, dimpled, rotors are to allow gasses to escape, correct.

drilled rotors started out as a weight saving measure on race vehicles (look at any modern motorcycle or high perf car), but was used later as a way to cheaply vent rotors. It's cheaper to drill than dimple or slot.

In terms of drilling for cooling, it's really minimal. As posted earlier, look up high end pads, they'll provide the best bang for the buck.

dimpled rotors are the strongest "altered" rotor, with slotted real close. Drilled rotors are real prone to warping. I used to race Vintage motorcycles, and drilling saved about 1lb/rotor, but I'd go through a set every other race, that's an extreme example, but the premise is there...