The Founding Fathers, the framers of the Constitution, wanted to form a government that did not allow one person to have too much authority or control. While under the rule of the British king they learned that this could be a bad system. Yet government under the Articles of Confederation taught them that there was a need for a strong centralized government.
With this in mind the framers wrote the Constitution to provide for a separation of powers, or three separate branches of government. Each has its own responsibilities and at the same time they work together to make the country run smoothly and to assure that the rights of citizens are not ignored or disallowed. This is done through checks and balances. A branch may use its powers to check the powers of the other two in order to maintain a balance of power among the three branches of government.
The three branches of the U.S. Government are the legislative, executive, and judicial. A complete diagram of the branches of the U.S. Government may be found in the U.S. Government Manual.