Law is a system of rules that governs the behaviour of people in a society and can be enforced by the state through courts and police. It has been described as an art, a science and a moral philosophy, and its precise definition is the subject of much debate.

The most obvious function of law is to define the rights and duties that people have towards one another, allowing individuals to make contracts and to own and control property. It also establishes a framework within which citizens can live together peacefully and to protect them from the threat of attack by other people or states. This framework can include the right to free speech and freedom of assembly, to education and to religion. The legal system also allows citizens to be protected against injustice, crime and war.

There are many different laws, which reflect the diversity of human societies and their needs. These differ from culture to culture, but there are certain common features in most systems. For example, most countries have some form of contract law that regulates the exchange of goods and services, while most states have a property law to set out people’s rights and responsibilities toward their possessions.

Laws are generally made and enforced by governments, which are usually called nation-states. They are elected (chosen) by the people to represent them, and to give them power to make laws that control the way that people can live, work and trade with each other. These laws may be enacted by a legislature, which results in statutes; by the executive, with decrees and regulations; or by judges through case law and precedent. In some countries, a combination of the three forms is used.

In the early modern period, philosophers argued about the role of morality in law. Utilitarians such as Jeremy Bentham argued that law should be seen as a set of commands, backed by the threat of sanctions, from a sovereign who people have a habit of obeying; natural lawyers, such as Jean-Jacques Rousseau, saw laws as reflecting innate moral principles, which could not change. This approach was widely accepted until the 19th century, when Max Weber and others reshaped thinking about the extension of state power, and contemporary concerns about government transparency and accountability have led to many changes in lawmaking processes.

There are a variety of law schools and professional bodies that teach and research law. These professionals are often known as attorneys or jurists. They can specialise in areas such as contract, tort, commercial and criminal law. They are often qualified to practice law in the country or jurisdiction where they work, but some of them can be trained to practise internationally.

Some types of law are complex and require a great deal of study and training to understand fully. For example, to determine what the law is on a particular issue, it is necessary to read all relevant statutes and cases, and to extract principles, analogies and statements by judges that explain how they would rule on similar issues. More recent decisions and those by higher courts carry more weight than earlier ones, but even the most expert lawyer can be wrong about a particular set of facts.