I'm not the best at trying to explain this sort of thing but it is down to an aircrafts tendancy to 'weather-vane'. For example, a cross-wind like yesterdays was blowing right to left across the runway. By blowing against the fin of the aircraft, it causes the tail to move downwind around the pivot point (the gear) which in turn causes the nose of the aircraft to turn into wind (i.e to the right). In that example you would apply left rudder to keep the nose straight, and right aileron to prevent the into-wind wing from being lifted by the wind.
A quick search on google threw these up if it explains it any better:
Section 13.5
Takeoff [Ch. 13 of See How It Flies]
and section 8.11
Yaw-Wise Torque Budget [Ch. 8 of See How It Flies]
Hope that explains it
