Chaps
You are straight and level and want to turn left. You put the left aileron up a little and the right aileron down. This slightly reduces the overall lift on the left wing and slightly increases that on the right. This imbalance makes the aircraft roll to the left. As soon as you get the bank angle you want you centralise the controls and the roll stops.
During the time the ailerons are not neutral the side with the down aileron has more drag unless the designer has used one of the tools at his disposal to keep the drag equal both sides when the ailerons are deflected.