Popping is one of the original funk styles of dance that came out of Fresno, CA in the 1970s. It is based on the technique of quickly contracting and relaxing muscles to cause a jerk in the dancer's body, referred to as a pop, tick or a hit. This is done continuously to the rhythm of a song in combination with various movements and poses. A popping dancer is commonly referred to as a popper.
Locking is a style of dance that originated in LA in the late 60s. It is distinctive for its very distinct stops, however these are complete stops in motion rather than contraction and relaxation of the muscles like in popping. It is generally more relaxed and movements are looser than those utilized in popping. Locking is typically danced to funk music.