Common law has three distinct meanings: the common law legal system, as contrasted with the civil law legal system; common law, as contrasted with statute law; and common law, as contrasted with equity.
The law developed in historical England mainly from judicial decisions which were based in tradition, custom and precedent. It may be written or unwritten in statutes or codes. The common law as applied in civil (as distinct from criminal) cases was devised as a means of compensating someone for someone else's wrongful acts (whether intentionally or by negligence [carelessness]). By the 17th century, the judicial system recognized that certain wrongs such as trespass were not susceptible of compensation because the wronged person didn't care about the money but wanted his land back (trespass) or the benefit of his bargain (that house he agreed to purchase) and the compensation could not make him whole. From this, courts of Chancery developed in England with a purpose of doing what was fair ("equitable") and through these courts one could force the wrongdoer to sell him that house or get off his land (or go to jail if he still refused). These systems of law (money damages) and equity (fashioning a remedy to fit the situation) continued as parallel systems and courts well into the 20th century.
It constitutes the basis of the legal systems of: the United Kingdom (except Scotland), the United States (except Louisiana), Canada (except Quebec), Australia, New Zealand, South Africa, India, Singapore, and many other countries. (Basically, every country which has been colonised at some time by Britain.)
The main alternative to the common law system is the civil law system, which is used in Continental Europe, Quebec, Louisiana, the former Soviet bloc, and most of the rest of the world. Scotland is often said to use the civil law but in fact it has a unique system which combines elements of the civil law and of the common law.
Statutes which reflect English common law are understood to always be interpreted in light of the common law tradition, and so may leave a number of things unsaid because they are already understood from pre-existing case law and custom.
By contrast, some laws are purely statutory, and may create a new cause of action. An example of this would be the tort of Wrongful Death, which allows certain persons, usually a spouse or child or estate, to sue for damages on behalf of the deceased. There is no such tort in English common law; thus, any state that lacks a wrongful death statute will not allow a lawsuit for the wrongful death of a loved one. Where a wrongful death statute exists, the damages or compensation available are limited to those outlined in the statute (typically, an upper limit on the amount of damages). Courts generally interpret statutes that create new causes of action narrowly - that is, limited to their precise terms - because they have no common law precedent for guidance.
Where a tort is grounded in common law, then all damages traditionally recognized historically for that tort may be sued for, whether mentioned in current statutory law or not. For instance, a person who sustains bodily injury through the negligence of another may sue for medical costs, pain, suffering, loss of earnings or earning capacity, mental and/or emotional distress, loss of quality of life, disfigurement, and more. These damages need not be set forth in statute as they already exist in the tradition of common law. However, without a wrongful death statute, most of them are extinguished upon death. An old saw (in states with no or low wrongful death damages) stated that it was better to back up and over the person to insure his death and limit your exposure!