The Constitution separates the power of government into three branches: the legislative power is vested in the Congress, the executive power rests with the President and his bureaucracy and the judicial power is granted to the Supreme Court and other federal courts. Whereas each branch of government has separate and particular powers as listed in the Constitution, each branch is also given the power, duty and ability to control and balance the other(s) in a system of checks and balances.
The Constitution grants all legislative power to the Congress. The Congress is bicameral and a bill has to pass both houses: the House of Representatives and the Senate. In this way the houses check and balance each other. Furthermore, both the executive and the judicial branch check and balance the Congress' legislative power. Although only the Congress can make laws, the President has the power to veto bills, in which case the bill can only pass with a 2/3 majority in both houses. Finally, if the Congress and the President agree on a law, the Supreme Court has the power of interpreting the laws and a power of review, i.e. the Supreme Court can declare a law unconstitutional and therefore void. Besides law making, the Constitution lists a wide range of congressional powers, I don't have the space to mention them all, but some of them will be analyzed below.
As chief administrator the President is required to see that laws are carried out, to enforce existing politics and to managing the bureaucracy. The president nominates the heads of the executive branch's departments, but these appointments are subject to the Congress' approval.
The constitution makes the president and Congress share powers in matters such as foreign policy and the US armed forces. The President is named the Commander in Chief of the US armed services and navy, but only Congress can declare war and approve funds of the military. However, this has not stopped