Agency law is a recent development in the Federal law. During the Great Depression Congress created agencies to assist in the recovery of the United States economy. While the agencies did help with the recovery the question of what law governed their actions and reactions to their creation by business prompted the passing of the Administrative Procedure Act which codified the law governing the agencies.
Agencies execute the laws that Congress passes, but they also have the power to create rules of their own through enabling statutes that are passed by Congress. They can also adjudicate issues regarding their rules, much like the judicial branch.
The rules and regulations that the agencies create are gathered together and published in the Code of Federal Regulations.
Some commentators have gone as far as to label the broad collection of agencies as the ‘fourth branch’ of the Federal government.
Under the Constitution the majority of the power of the Executive branch of government is vested in the President. The President is the commander of the United States military and the chief diplomat for the United States for international affairs. The President has the unique powers of being able enter into international treaties, veto acts of congress, and appoint Supreme Court Justices. The President is charged with enforcing the laws of the United States and ensuring the defense of the nation.
Under the President is the Vice President. The Vice President assists the President administer the Executive branch and becomes the President should something happen to the President that would render them unable to perform their duties. The Vice Presidential role is considered as part of the Executive branch, however the Constitution does not specifically name which branch of the government that the office is tied to.
The President does not enforce the law alone. The Executive branch has a variety of Cabinets, executive departments, agencies, and government corporations that assist the President to ensure that the laws of the United States are followed.
Under the Constitution, the President is granted special powers, called executive powers, that no other member of the Federal government has. Among these powers are the abilities to remove ambassadors, appoint and remove executive personnel, and enter into executive agreements with foreign powers.
For a more complete listing of the executive powers granted to the president and examples of how they can be used, you can use this link.