President of the United States(6)

Oath of office and term of office
The President's term of office begins at noon on January 20 of the year following the election. This date, known as Inauguration Day, marks the beginning of the president's and vice president's four-year terms. Before assuming office, the president-elect is constitutionally required to take the presidential oath:
“
I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States.[5]
”
Although it is not required, Presidents traditionally used a Bible to take oath of office (some exceptions: Chester A. Arthur in 1881 and Theodore Roosevelt in 1901 did not use a Bible, while Lyndon B. Johnson in 1963 used a Roman Catholic missal, because there were no Bibles aboard Air Force One).
Although no law requires that the oath of office must be administered by someone, new presidents are traditionally sworn-in by the Chief Justice of the United States. George Washington was sworn-in by Chancellor of New York Robert Livingston while John Tyler, Millard Fillmore, Chester A. Arthur, Theodore Roosevelt, Calvin Coolidge and Lyndon B. Johnson by lesser-capacity judges.
George Washington, the first president, set an unofficial term limit of two terms, which was generally followed by subsequent presidents as precedent. After the twelve-year presidency of Franklin D. Roosevelt, who was elected four times, but died shortly after beginning his fourth term, the Twenty-second Amendment to the Constitution was ratified, barring presidents from being elected more than twice, or once if they served more than half of another president's term. Prior to Roosevelt, several presidents had campaigned for a third term, but none were elected. Harry S. Truman, who was president at the time of the amendment's ratification and thus not subject to its terms, also briefly sought a third term before withdrawing from the 1952 race, although it would have been only his second full term as president.
Since the amendment's ratification, three presidents have served two full terms: Dwight D. Eisenhower, Ronald Reagan, and Bill Clinton. Richard Nixon was elected to a second term, but resigned before completing it; George W. Bush will become the fourth upon completion of his current term on January 20, 2009. Lyndon B. Johnson was the only president under the amendment to be eligible to serve more than two terms in total, having only served for 14 months following John F. Kennedy's assassination. However, he chose not to run in the 1968 election.
Removal from office
See also: Impeachment in the United States and United States presidential line of succession
Vacancies in the office of President may arise because of death, resignation, or removal from office. Articles One and Two of the Constitution allow the House of Representatives to impeach high federal officials, including the president, for "treason, bribery, or other high crimes and misdemeanors", and give the Senate the power to remove impeached officials from office, given a two-thirds vote to convict. Two presidents have thus far been impeached by the House, Andrew Johnson in 1868 and Bill Clinton in 1998. Neither was subsequently convicted by the Senate; however, Johnson was acquitted by just one vote.
Per the Twenty-fifth Amendment, the vice president and a majority of the Cabinet may suspend the president from discharging the powers and duties of the office once they transmit to the Speaker of the House and the President pro tempore of the Senate a statement declaring the president's incapacity to discharge the duties of the office. If this occurs, then the vice president will assume the powers and duties of President as Acting President; however, the president can declare that no such inability exists, and resume executing the Presidency. If the vice president and Cabinet contest this claim, it is up to Congress, which must meet within two days if not already in session, to decide the merit of the claim.
The United States Constitution mentions the resignation of the president but does not regulate the form of such a resignation or the conditions for its validity. By Act of Congress, the only valid evidence of the president's decision to resign is a written instrument declaring the resignation signed by the president and delivered to the office of the Secretary of State.[6] The only president to resign was Richard Nixon on August 9, 1974; he was facing likely impeachment in the midst of the Watergate scandal. Just before his resignation, the House Judiciary Committee had reported favorably on articles of impeachment against him.
The Constitution states that the vice president is to be the president's successor in the case of a vacancy. If both the president and vice president are killed or unable to serve for any reason, the next officer in the presidential line of succession, currently the Speaker of the House, becomes acting president. The list extends to the President pro tempore of the Senate after the Speaker, followed by every member of the Cabinet in a set order.
Source : http://en.wikipedia.org/wiki/President_of_the_United_States