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

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