Law is a system of rules that regulates behavior in a community and is enforced by a controlling authority through penalties. It includes both formal legal codes and informal customs and policies enforceable by judges in the courts. Laws are usually derived from prior judicial decisions; a compilation of such decisions is called case law. Some countries, such as Japan, have a civil law system in which laws are based on written constitutions and other explicit codes of procedure.

Some general categories for law include contract law, which regulates agreements; criminal law, which deals with crimes against a person or the property of persons; and family law, which addresses marriage, divorce, children and custody. The law also governs such matters as property ownership and taxes.

The law in a society can have many functions, including keeping the peace, maintaining the status quo, preserving individual rights, and facilitating social change. In some cases, a government’s power to make and enforce law can be used to oppress minorities or opponents. The purpose of the law may be defined by the political structure of a country, and such definitions are often the subject of debate.

A person trained in the law is called a lawyer or jurist. Law students generally specialize in one of these areas, though some study a broad range of legal subjects, including the history of law and legal philosophy. Other important topics include constitutional law and international law.

Law is the source of all civil and criminal justice systems. It is the foundation of a society, and some people make careers of advising others about it or representing them in court. Other people work in the administration of justice, such as prosecutors, public defenders and judges. The philosophies that shape the law are important, and these are described in articles about the judicial system; censorship; crime and punishment; and war. A complete study of the law requires more than an article, and books about it are listed in the bibliography of law. See the articles on legal profession, legal education and training and law school for further information. A related topic is the philosophy of law, which discusses the nature and purpose of laws and legal systems in general. The article on legal systems describes the main types of laws in use around the world. This article also explains the relationship between the law and other fields of knowledge, such as biology. For a comparison of different systems of law, see comparative law. For the earliest discussions of laws in the Western world, see canon law and Roman law. For laws in other religions, see Islamic law and Jewish law. For more on the evolution of law, see jurisprudence. For the relationship of law to other social issues, see human rights; land reform; and social justice. Also see the articles on religion; and law and politics.