The founding fathers wanted the USA to have limited government. To ensure the government would not become over-powerful, they divided power between three institutions: the presidency, Congress and, although the role was not clearly defined, a Supreme Court.

These were the executive, legislative and judicial branches of government, respectively.

This division has been portrayed in different ways. In a celebrated phrase, Professor Richard Neustadt wrote in the 1960s of 'separated institutions sharing power'. All three branches, for example, contribute to the legislative process. While Congress has the main responsibility for making law, the president can veto proposed legislation. Furthermore, the Supreme Court can, in some instances, declare a law unconstitutional and therefore invalid.

However, the US constitutional system is more commonly described as a separation of powers. Although there are ties and connections, each branch of government has its own sphere of responsibility. There are, furthermore, checks and balances. These reinforce the separation of powers by ensuring that each branch of government plays a part in watching over the work - or checking the power - of the other branches. This not only places limits on the overall power of government (by balancing the different branches) but also ensures that the overall government functions effectively and efficiently. 

For example, although the president appoints federal judges and senior government officials, the Senate must confirm his or her nominations. This curtails the president's powers but is also intended to ensure that the people nominated are of a high calibre. 

Federal Government Power

Legislature makes the laws - Congress makes the laws

Executive carries out the laws - President carries out the laws

Judiciary enforces the laws and interprets the laws - Supreme Court enforces the laws