The most important doctrine on which the American Constitution is based, is the Separation of powers: the executive (hold by the Presidency), the legislative (kept by the Congress) and the Judiciary (detained by the Courts). So each government institution has got limited and specific powers and the duty to check one other, to avoid the abuse of power.
Government power is also limited by the dual system of government which shares tasks and responsibilities between the government as federal and the individual State.
The Constitution was ratified by all 13 States in 1791 and contained the Bill of Rights (10 amendments which protect citizens against the possible tyranny by the federal government)._so far 16 amendments have been added so now they're 26_
The president is elected for a term of 4 years and can be re-elected only one more time. Previously, the President function was little more then ceremonial but now the importance of the Presidency over Congress has increased.
The president proposes a full legislative programme to Congress, but the congress can also have no intention to cooperate with him. In addiction, even when a law passes, it's difficult to ensure that it's effectively implemented by the federal bureaucracy. So the President has basically the power to persuade.
The Vice-President's duty is presiding over the debates in the Senate. He takes over from the President in case of death, resignation or sickness. Now the Vice-President tasks has grew especially in foreign affairs.