adverse yaw is caused by the use of ailerons, and the rudder is used to overcome this. adverse yaw is more noteable at slower speeds. let's say you want to turn left. in order to do so, you roll your aircraft to the left until you have established the desired bank of your wings. once you have done this, you typically ease off the airleron pressure so the aircraft doesn't keep rolling.

ok, now when you first apply the aileron to roll left, making the right aileron moving down, and the left one moving up, it changes the angle of attack on both wings. The right wing, with the dropped aileron, is now creating more lift because the aileron has increased the right wings effective camber (curvature of the wing). Doing so has increased the drag on that wing, particularly what's known as induced drag. the concept of induced drag is hard to explain thru text without a diagram. to put it short, when lift is increased, so is drag typically.

so, when making the left turn, the nose of the aircraft may yaw to the right initially when applying left stick. in order to overcome this, you can apply some left rudder to swing the back of the aircraft out to the right, wich yaws the nose to the left and prevents the adverse yaw from ever happening.

Hope this helped... anyone else feel free to add/correct anything i've said.