Forward Slip - where you are keeping the longitudinal axis of the aircraft in line with a fixed point, flying straight in relation to the ground.
i.e. point the nose at the runway with rudder and counteract the drift with opposite aileron. This is good for a cross wind approach as you can just land on the into wind wheel without having to straighten up. (in other words a standard wing down approach)
Side Slip - your target direction is out to the side of the windscreen. i.e. you are pointing (heading) at 30 degrees to your track, or flying sideways in relation to the ground.
hope this makes sense