The delegates to the Constitutional Convention faced a difficult challenge. They wanted to ensure a strong, cohesive central government, yet they also wanted to ensure that no individual or small group in the government would become too powerful. Because of the colonies’ experience under the British monarchy, the delegates wanted to avoid giving any one person or group absolute control in government. Under the Articles of Confederation, the government had lacked centralization, and the delegates didn’t want to have that problem again. To solve these problems, the delegates to the Constitutional Convention created a government with three separate branches, each with its own distinct powers. This system would establish a strong central government, while insuring a balance of power.
Governmental power and functions in the United States rest in three branches of government: the legislative, judicial, and executive. Article I of the Constitution defines the legislative branch and vests power to legislate in the Congress of the United States. The executive powers of the President are defined in Article 2. Article 3 places judicial power in the hands of one Supreme Court and inferior courts as Congress sees necessary to establish. A complete diagram of the branches of the U.S. Government (PDF, 9.7k) may be found in the U.S. Government Manual.
Though in this system of a "separation of powers" each branch operates independently of the others. However, there are built in "checks and balances" to prevent tyrannous concentration of power in any one branch and to protect the rights and liberties of citizens. For example, the President can veto bills approved by Congress and the President nominates individuals to serve in the Federal judiciary; the Supreme Court can declare a law enacted by Congress or an action by the President unconstitutional; and Congress can impeach the President and Federal court justices and judges.