Opinions will probably vary, but here's mine:
A side slip is what you use in a cross-wind approach - wing down & top rudder to keep the plane lined up on the runway. Can be held into the flare.
A forward slip is specifically to blow off height in a hurry - full flaps, nose down, wing-down and lots of top rudder. The aircraft still tracks towards the landing point but the fuselage is not in line with it. Airspeed must be kept higher (75-85kts in the little Robin I was in). Must straighten out & slow down before landing flare.
http://en.wikipedia.org/wiki/Slip_(aerodynamic)