FWD wheels compared to RWD wheels. whats the difference

uh if you're referring to front wheel drive vs. rear wheel drive then it's the front wheels move the car instead of the rear and on rear wheel drive cars, the rear tires make the car move.

if you're referring to some type of rims for FWD cars, there's no such thing. a rim is a rim, a tire is a tire. i dunno if this is what you meant, but im gonna stop now :(

edit: maybe you're referring to the drivetrain. the tranny spins the driveshaft which then turns the gears and rotates the axels on a RWD car, no idea on a FWD :shrug:
 
backspacing varies from vehicle to vehicle, FWD or RWD.

20" rims can fit on an SUV which is RWD and they can also fit on a honda civic. i guess the answer to this question is backspacing, not what brand/style.