United States Constitution: Article II, Section 4: 

• The President, Vice President and all Civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.

• Impeachment is the ultimate power of Congress to deter and to punish abuse of power by officers of the executive and judicial branches. Federal judges constitute the greater number of impeached and convicted officers. President Andrew Johnson won acquittal by a single vote, and President Richard Nixon resigned before he could be impeached.  President Bill Clinton was impeached by the House and acquitted by the Senate.

• http://www.senate.gov/civics/constitution_item/constitution.htm#a2_sec4

The Senate's Impeachment Role

• The United States Constitution provides that the House of Representatives "shall have the sole Power of Impeachment" (Article I, section 2) and that "the Senate shall have the sole Power to try all Impeachments .... [but] no person shall be convicted without the Concurrence of two-thirds of the Members present" (Article I, section 3). The president, vice president, and all civil officers of the United States are subject to impeachment.

• The concept of impeachment originated in England and was adopted by many of the American colonial governments and state constitutions. At the Constitutional Convention in 1787, the framers considered several possibilities before deciding that the Senate should try impeachments.

• Impeachment is a very serious affair. This power of Congress is the ultimate weapon against officials of the federal government, and is a fundamental component of the constitutional system of “checks and balances.” In impeachment proceedings, the House of Representatives charges an official by approving, by majority vote, articles of impeachment. A committee of representatives, called “managers,” acts as prosecutors before the Senate. The Senate Chamber serves as the courtroom. The Senate becomes jury and judge, except in the case of presidential impeachment trials when the chief justice of the United States presides. The Constitution requires a two-thirds vote of the Senate to convict, and the penalty for an impeached official is removal from office. In some cases, disqualification from holding future offices is also imposed. There is no appeal.

• http://www.senate.gov/artandhistory/history/common/briefing/Senate_Impeachment_Role.htm