The first three articles of the United States Constitution establish three equal branches of government. This concept of equal power to all branches is maintained by a system of checks and balances. This system was incorporated into the U.S. Constitution to ensure that each of the three branches maintain an equal level of power within the government.
It is the responsibility of each independent brach of government to check the other branches when they attempt to encroach on the power afforded to that branch in the Constitution. These checks include the congressional power of the purse, the executive power of veto, and the judicial power to interpret established legislation.