Liability insurance policies are either "occurrence-based" or "claims-based." An occurrence-based policy provides insurance coverage for a loss that "occurred" during the policy period, no matter when the claim is brought against the insured. A claims-based policy provides coverage for a claim that is brought within the policy period, no matter when the loss occurred.
Generally speaking, auto policies, homeowners policies, and commercial general liability policies are occurrence-based. Many professional liability policies are claims-based.
Let's say you had a policy that provides coverage for injury or damage caused by an apple tree you own. The policy was in effect from June 30, 2006 to June 30, 2007.
A plaintiff claims that as a result of your negligence, a branch of the apple tree broke and hit her on the head, injuring her.
If you had an occurrence-based policy, your insurance will cover you if the accident happened between June 30, 2006 and June 30, 2007. It doesn't matter if you were not notified of the accident until January, 2008; the insurance will still cover you.
If you had a claims-based policy, your insurance will cover you if you are notified, and notify your insurance company, of the accident between June 30, 2006 and June 30, 2007. If the accident happened between those dates but you don't receive notice of it until January, 2008, your policy will not cover you.
If future posts I will discuss the various issues that arise with respect to what "occurrence" means in an occurrence-policy, and the precautions you should take if you have a claims-based policy to make sure you do not have any gaps in coverage.