Constitutional law is the branch of law establishing the basic principles and laws of a nation, state, or social group that determine the powers and duties of the government and guarantee certain rights to the people in it.
As its name implies, Constitutional law is spelled out in the Constitution itself. The courts -- which are created by the Constitution -- have the ability to interpret what the Constitution means. The courts exercise this power by rendering decisions in cases and by reviewing the decisions of lower courts. The legislature - which is also created by the Constitution - has the ability to impose some limits on the courts. This separation of powers is thought to provide a system of checks and balances.
Constitutional law often involves actions taken by state or local governments and separation of powers between the three branches of government. Constitutional challenges often involve claims of discrimination and unlawful takings. These challenges often relate to due process and equal protection violations. Age, race, gender, national origin, religion, and other freedoms or limitations are often challenged.