The United States of America is a constitutional republic that includes federal and state legal systems. The federal system comprises three branches of government: the legislature (Congress), which enacts laws, the executive branch (headed by the President), which enforces the laws, and the judicial branch (including the Supreme Court), which interprets the laws. The US Constitution grants distinct powers to each of the three branches, and establishes a system of checks and balances designed to limit the influence of any one branch. The governments of the 50 states, the District of Columbia, and various territories follow the federal three-branch model.
The Constitution establishes a federal government of enumerated powers; the powers not delegated to the federal government are reserved to the states or to the people. The Supremacy Clause of the Constitution provides that federal law supersedes any state law. Thus, each state is empowered to pass its own laws, provided that they do not conflict with the US Constitution or federal law. States also may establish subordinate, local government entities, such as counties, towns, and municipalities, which in turn may enact their own laws, and perform executive and judicial functions, as a component of the state system.