Whiplash injuries are the generic word for an injury to your soft tissue (tendons, muscles and ligaments) caused by a sudden acceleration deceleration force. In the usual situation, most of the whiplash injuries occur from a rear end collision.
The unfortunate part is that most soft tissue injury claims are inherently subjective in nature. That means that there are not a lot of physical signs (herniated disc, broken bones, etc.) which support that an injury has occurred. Rather, much of the injury is based on what you may be telling your doctor or therapist about your pain from the injury.
Usually, whiplash injuries are categorized as mild, moderate or severe. Unfortunately, doctors and therapists vary greatly on what injuries they place in each category as do the various ICBC adjusters.