Whiplash describes any injury of the neck's soft tissues resulting from forced movement.
The head's jerking action (not to be confused with the jerk who rear-ended you) pulls muscles and ligaments beyond their typical range of motion , causing fibers to tear. Torn fibers in a muscle or tendon is called a strain, whereas in a ligament it is called a sprain. Whiplash often involves both.
Extreme cases may involve fractured or dislocated bones, too. And if you're the jerk doing the rear-ending, you may experience severe litigation.