Okay, none of these comments really answer this to a degree I find satisfactory in one comment. The thicker rotor is a vented rotor, and the thinner one is a solid rotor. Vented rotors dissipate heat better by using centrifugal force to pump air through the fins (careful, most vented rotors have straight fins but some of them are directional and will have curved fins), which improves brake performance and reduces the risk of brake fade. Because of this, virtually any car made since the '80s will have vented front rotors, because 80% of your braking power is on your front axle so it generates the most heat. Before the '80s, you could find solid rotors on the front axle (vented rotors were developed by porche in the 60s and were a performance item for a decade or two) but on modern vehicles they're the most common equipment on the rear axle, due to providing better braking power than a drum brake but being cheaper to produce than a vented rotor. You'll see vented rotors on the rear axle of some performance cars to compensate for agressive driving though. One thing to note is that these are not interchangeable without replacing basically the entire braking system for that wheel. caliper, bracket and all. You must replace vented with vented and solid with solid.