Its all size, a class 3 is about a 4" square chunk of Iron with much bigger mounts. The class 2 goes down to about 1"-1.5" alll around even the receiver, and a class 1 is basically almost nothing the receiver itself is a flat peice of metal that slides into a slot. never really dealt with them only seen them. The only real reason for having the different classes is for smaller cars because if you tried to stick a class three under your car it would probably drag the ground, or atleast when you hit a bump Im sure it would. Im not really sure about the number of bolts, because I have seen many class threes that were different in that aspect, anywhere from 4 bolts to 16, depending on the application.