In any negligence-based lawsuit, such as one involving a car accident with another driver, in addition to showing that you have been harmed in your person, your property or both you will need to prove that the defendant or defendants caused your injury. But the law in California when it comes to personal torts treats causation as a two-part consideration, involving “actual” and "proximate" cause.
California courts use proximate causation to limit the scope of the damages that a plaintiff can claim. Only that harm which could have been foreseen at the time the original injury are generally allowed for the jury to consider. This foreseeability determination depends upon what risks to the plaintiff the defendant could reasonably have anticipated as a direct result of his acts.
How are these different from each other?
Actual cause: Actual causation, also known as cause-in-fact, is ordinarily the easier element to identify. As a plaintiff, you will need to show that you would not have suffered the injury but for some action on the part of the defendant. Alternatively, in some cases you will need to show that the injury was caused by the failure of the defendant to take a necessary action.
Proximate cause: Proximate causation is perhaps best explained not so much as a way of establishing the defendant's liability -- this is more in the realm of actual cause -- but rather of limiting it. Once the chain of events that causes harm is started, it can lead to a follow-on effect of additional, yet unanticipated events that might also indirectly cause harm even though they are not directly linked to the act that caused the harm to you.
This post is meant to provide an introduction to the concept of proximate causation, and is not a comprehensive treatment of the subject. As such, you should not read it as specific legal advice. If you have questions about a possible negligence claim, a personal injury plaintiff's attorney can help you to understand how causation will factor into your considerations, as well as what the other elements of a cause of action will be.