The Supreme Court is in session from October to June. One of the most important duties of the justices is to decide whether laws passed by the Congress agree with the Constitution. If the Supreme Court decides that the Constitution does not give Congress the power to pass a certain law, the court declares the law to be unconstitutional. Such a law can no longer be enforced by the President and his executive officers.
In addition to federal courts, each state has its own judicial system, including its own Supreme Court. This means, that the United States has two systems of law, state and federal. As a matter of fact, the country has 51 sets of courts (in fifty states and the District of Columbia). As a result the USA has the most complex judicial system in the world.
The federal and state courts have the power to declare legislative acts unconstitutional. By Constitution the courts have the power to hear and decide the two classes of cases - criminal and civil.