Do you know the difference between solid and vented rotors?  If you're thinking this question is about the drill-pattern on the rotor surface, it's not.  What this question is referring to is the inner structure of the friction surface.  

A solid brake rotor is a single, solid disc.  Vented rotors have two solid discs, divided in the middle by structures called vanes.   Vanes add material to a rotor, increasing the rotors thermal capacity and creating additional pathways where heat can escape.

It's most common to see vented rotors on the front brake system of automobiles and solid rotors on the rear brake system.  The front brakes do more work as most of the vehicle's weight is transferred to the front when slowing down.                                                                      ...