The common law, sometimes known as case law, is a legal system in which previous court decisions establish legal precedents & new decisions must be in keeping with past decisions. Common law may also refer to a legal system which relies on common customs and usages, rather than on codified written laws, or statutes. The common law forms a major part of the legal systems of regions in the world that are or were formerly under the rule of the British Empire, including the United States, with the exception of Louisiana, which takes its model from civil law, or codified decree, in the style of the original French governors.
While judges develop the common law gradually, through the force of their rulings in specific cases, it won't be found in any written form comparable to statutory law. Common law is instead the common consensus of legality and illegality that developed over centuries of individual court rulings in Britain over roughly 800 years.
While the common law system is the departure point for the criminal law in the United States, the United States Constitution gives legal precedence to whatever statutory laws the various state and local jurisdictions enact. At this time, most of the criminal law in the United States is codified into statutes. However, many states still directly recognize elements of the common law and, further, it is sometimes necessary to go back to the common law to discover the intrinsic meaning behind a statutory law. For example, a statute may say that negligent homicide is punishable by one to three years in prison without actually defining homicide. In this case, it would be necessary to go back to the common law for the definition.
State legislatures are permitted to pass what laws they see fit, as long as these don't contradict federal statutes, and local legislatures are permitted to pass what laws they see fit, so long as they don't contradict state or federal laws. Declaratory and remedial statutes are frequently passed to reinforce, expand or restrict the common law in the myriad jurisdictions of this country, and the legal landscape can vary considerably from state to state and town to town.
The strength of common law system lies in its flexibility, and the group consensus of judgement that it represents. Because the whole of common law is modified slightly by every court ruling, it is believed to better represent the hearts and minds of the people it seeks to govern.