The impeachment process begins with the House of Representatives investigating whether there are sufficient grounds to impeach the president; if it determines that there are, an impeachment trial takes place within the Senate. There are no instances of an American President being forced out of office due to impeachment, and only four instances in which there was legitimate debate in Congress about potentially impeaching a president, as of 2015.
Initially, the House Judiciary Committee chooses whether to pursue impeachment, followed by the Judiciary Committee initiating a formal inquest into the subject of impeachment. Depending on the outcome of the inquiry, the Judiciary Committee sends a resolution to the House either indicating that impeachment is warranted and supports the conclusion with the Articles of Impeachment, or that there is insufficient reasoning to support impeachment.
After debate, the Full House votes on each Article of Impeachment. If a simple majority approves of any one of the Articles, the President is officially impeached. However, this is merely a way of charging the President with a crime; a trial still takes place in the U.S. Senate, and the President is represented by lawyers. The Chief Justice of the Supreme Court serves as prosecutor and the 100 Senators act as jurors.
After a trial, the Senate meets privately to decide upon a verdict. A 2/3 vote results in a conviction. Next, the Senate decides whether to remove the President from office, and may also vote on whether the President can hold any public office in the future.