If you have been in a traffic accident, the law holds the responsible party liable and compensates the injured party. Recovery or liability can be based on negligence, recklessness, or product liability. In the simplest of cases, your insurance company will handle the matter, compensating the injured party.
Unfortunately, not all cases are so simple. If you have been in a traffic accident, you are best advised to write down all the information you can and take subsequent notes on any harm you suffered and their daily affects.
In many states, you will need to report the accident to your state DMV as soon as possible. Do so, but give only a brief statement, and do not in any way implicate yourself by admitting liability. A determination of who was at fault will depend on who was the careless driver based on traffic laws. Where there was a traffic violation, there is a stronger presumption that the traffic violator was at fault. However, where there were no major traffic violations, a court or insurance company will have to determine who was at fault based on the accounts of the drivers and eyewitnesses, if there were any.
If you were hit from behind, however, it is nearly always the other drivers fault, because traffic laws require that drivers must maintain a certain distance from the automobiles in front of them to be able to stop in time. Moreover, cars making a left turn are nearly always liable to cars coming straight in the other direction unless the car going straight was going to fast or ran a light.