Liability insurance (also called third-party insurance) is a part of the general insurance system of risk financing to protect the purchaser (the "insured") from the risks of liabilities imposed by lawsuits and similar claims and protects the insured if the purchaser is sued for claims that come within the coverage of the insurance policy.
Originally, individual companies that faced a common peril formed a group and created a self-help fund out of which to pay compensation should any member incur loss (in other words, a mutual insurance arrangement). The modern system relies on dedicated carriers, usually for-profit, to offer protection against specified perils in consideration of a premium.
Liability insurance is designed to offer specific protection against third-party insurance claims, i.e., payment is not typically made to the insured, but rather to someone suffering loss who is not a party to the insurance contract. In general, damage caused intentionally as well as contractual liability are not covered under liability insurance policies. When a claim is made, the insurance carrier has the duty (and right) to defend the insured.
The legal costs of a defence normally do not affect policy limits unless the policy expressly states otherwise; this default rule is useful because defence costs tend to soar when cases go to trial. In many cases, the defense portion of the policy is actually more valuable than the insurance, as in complicated cases, the cost of defending the case might be more than the amount being claimed, especially in so-called "nuisance" cases where the insured must be defended even though no liability is ever brought to trial.