When the car is speeding up , the wheels are rotating and have tendency to go backwards , hence the friction acts in the forward direction , giving a torque and acceleration. Without friction wheels wouldn't be able to rotate just as you wouldn't be able to sit in the car ! When brakes are applied , they stop start stopping the wheel , the wheel having a tendency to go forwards and hence the friction acts backwards hence stopping the car further. This is all right. But , I had once read when I was in 11th that when a car is accelerating friction acts in forward direction on 2 wheels (probably ,I had read the front ones) and in backwards direction on the other two. When I read this , it seemed contradictory. I don't know whether it was right or not but certainly , I had read this. If anyone could shed light on this too , it would be better.