did you seriously just ask why they have the classes ????
Let me brake this down for you...

AWD Class = All wheel drive vehicles hence AWD
FWD Class = Front wheel drive vehicles
RWD Class = Rear wheel drive vehicles

The reason why these classes are put into place is so that the vehicles of the same drive terrain can compeate together. Putting an AWD against FWD or RWD would be fucking stupid seeing that the car is going to handle much better than any of the other two. It makes the comp. more fair in between the rest of the classes within these groups.

As for your other question you are going to want something with little to no body roll and with great responsiveness. Your best bet for this would be to use the link stated above to do more research in to the field.