The crack of a whip is caused by a loop traveling along the whip itself. This loop gains speed until it reaches the speed of sound, and this creates a sonic boom.
It was previously thought that the crack sound results from the tip of the whip traveling fast enough to break the sound barrier and creating a sonic boom. However, new findings detailed in Scientific American magazine suggest otherwise. The article notes that even though some parts of the whip may travel at even greater speeds, it is the loop traveling down the whip that generates the sonic boom.