The U.S. government is mainly organized under two principles: the Separation of Powers and the System of Checks and Balances. Under the principle of the Separation of Powers, the power of the government is divided into three branches: the Legislative, consisting of a bicameral Congress that make laws, the Judicial, consisting of a Supreme Court whose main responsibility is to evaluate laws and has the power of Judicial Review, and the Executive whose power is vested in the President and its function is to enforce laws.
Likewise, under the system of Checks and Balance, each branch has the power to oversees, limit and respond to the action of the other. For example, under this system, the Legislative passes bills, however, these bills can not become laws right away: they first have to be approved by the President; simultaneously, the Judicial branch has the power to determine if a law or an act of Congress or of the President is unconstitutional.
The Founding Fathers created this form of government to prevent tyranny and to have a balance in government. With this model, the power is not held by only one institution but by three branchers that have the power to place limits in the others, and this helps prevent abuses of authority.