Law is a system of rules enforced by society to regulate behavior and ensure that everyone follows the same rules. Laws can be created by social or governmental institutions, but they must be agreed upon by all parties in a society for them to be valid and enforceable. Laws can be used to create a fair and equitable society, or they can be misused by those with more power or wealth to oppress minorities or limit the rights of others.

Law has many purposes, but four of the most important are establishing standards, maintaining order, resolving disputes and protecting liberties and rights. People who break the law may be punished by paying fines or being put into jail. The law may also set out the rights and responsibilities of citizens. It may be used to govern economic activity, such as banking regulation or laws about best practice for investment. It can be used to protect the environment, such as environmental laws or treaties, and it may be used to control military activities, such as international law or treaties governing the conduct of war.

It is often difficult to define law because laws differ from society to society. The most common definition of law is that it is a set of rules that form a framework to ensure a peaceful society. These laws can be enforced by mechanisms created by the state and sanctions can be imposed if they are broken. These rules can be created by a group of politicians in a legislature, such as a parliament or congress, elected by the people to represent them; by a single legislator, resulting in statutes; by the executive branch, in decrees and regulations; or judges in common law legal systems, where decisions are binding on lower courts by the principle of stare decisis.

Some philosophers have developed different theories of law. Roscoe Pound developed his own theory of law in which he said that law was a means of social control where conflicting social wants and interests battled for recognition. Another philosopher, Hans Kelsen, came up with a pure theory of law where he said that law is the result of observer participation, involving the participant assigning true or false values to mathematically undecidable propositions.

A lawyer or jurist is a professional who studies and argues the rules of law. They may specialise in a particular area of law, such as labour law or property law. They can help their clients with a range of problems, from disputes in their workplace to disputes with the government. They may also help their clients with planning and business advice. Some lawyers can also act as advocates for their clients in court, which is known as litigation. Other lawyers can deal with transactional matters, such as preparing contracts and dealing with tax issues. They are called solicitors and barristers.