Every American school child learns that the U.S. federal government is composed of three branches: the executive, the legislative and the judiciary. The legislative branch, made up of the House of Representatives and the Senate, creates the laws of the land; the executive puts those laws into practice and enforces them; and the judiciary takes a cold, hard look at the laws to make sure they're constitutional.
But, of course, things are bit more complicated than that. The executive branch, headed by the president, does indeed execute the laws created by the legislature, via the federal departments, boards, commissions and agencies it controls. The president is directly responsible for appointing the heads of the 15 different executive departments (including the Department of Defense, the Department of Transportation and the Department of Labor), in addition to hundreds of agencies (such as the Central Intelligence Agency, Environmental Protection Agency and the Federal Emergency Management Agency), boards (like the Federal Reserve Board) and commissions (e.g., the Securities and Exchange Commission) — not to mention nominating federal judges, Supreme Court justices and ambassadors.
As the person in charge of all of these organizations, the president can issue executive orders to them, and those orders automatically become law. While this is seen as a way for the president to effectively bypass the legislature, it's a method that has an important downside — namely that a successor can use that same executive power to reverse the orders of his or her predecessor.
So, for instance, when Barack Obama took office, his first executive order rescinded George W. Bush's executive order to make access to presidential archives more difficult.
As head of government, the president signs Congress's new legislation into law or vetoes it. Congress can override the veto but only with a two-thirds vote of both houses. The president is also, famously, the Commander in Chief of the country's vast armed forces and, as such, can take the country to war, although technically only with the permission of Congress (we'll come back to that technicality shortly).
The president is also the head of state, meaning that he or she is head of foreign relations and can negotiate trade deals and treaties, although here Congress really does have the last word and must ratify such agreements with the magic two-thirds vote.
And finally, the president can pardon anybody he or she wishes, as long as that person has been convicted of a federal crime [source: Whitehouse.gov].