A Whip Kick (also called a Whipping Kick or huryeo chagi) is a kick in which the kicking foot whips in a wide arc to strike the target, usually with the bottom or the back of the foot.
A Front Whip Kick is executed with one’s body facing the target. The knee is first bent and then and straightened, with the bottom of the foot swinging in an arc, up into the target.
A Rear Whip Kick is executed with one's body facing either partly or completely away from the target. In this case, it's often not necessary to bend the knee as part of the whipping motion.