understeer is when the front tires lose grip before the rear, in which case the car will under steer....oversteer is when the front tires lose grip before the front end, in which case the car will over steer...simple....lose front + stiff back = oversteer encouraged...lose back + stiff front = understeer encouraged...weight distribution front/rear also plays a part in oversteer/understeer...front wheel drive cars understeer, rear wheel drive cars oversteer....