The descriptions of the crab are correct. A slip is accomplished by using opposite aileron and rudder (right aileron, left rudder or vice versa) for wind correction and to hold the aircraft on centerline when landing in a crosswind. For example, if you were landing with a right crosswind, you would apply right aileron, and counter act that with left rudder to stop the turning tendency and hold the aircraft on the centerline of the runway until touchdown. A slip can also be used to lose alot of altitude quickly without increasing airspeed in full scale aircraft.