So, a crab is when the aircraft is headed one way, but is tracking or moving
Slipping, on the other hand, involved two bodyparts, hands on the controls (ailerons) into the wind while simultaneously applying opposite rudder with the foot. This is used for landing the a/c in crosswind, or if you want/need to descend more quickly than you are with the usual tricks (reducing AS, dumping flaps, etc.)
Ironically, although the experienced pilots were able to give me the textbook def (which I already knew), they couldn't tell me what to do with my body!!! As one guy put it, "when you just do it all the time naturally, you don't even think about what you are doing anymore", hehe. A fellow student pilot was best able to help me this time!