Background
Impeachment in the United States is an expressed power of the legislature that allows for formal charges against a civil government officer for crimes committed in office. The actual trial on those charges, and subsequent removal of an official on conviction on those charges, is separate from the act of impeachment itself.
The Senate as a Court of Impeachment for the Trial of Andrew Johnson
The Impeachment of Andrew Johnson
Impeachment is analogous to indictmentin regular court proceedings. Trial by the other house is analogous to the trial before judge and jury in regular courts. Typically, the lower house of the legislature will impeach the official and the upper house will conduct the trial.
At the federal level, Article II of the United States Constitution (Section 4), states that the President, Vice President, and all civil officers of the United States shall be removed from office on impeachment for, and in conviction of, treason, bribery, or other high crimes and misdemeanors. The House of Representatives has the sole power of impeaching, while the United States Senate has the sole power to try all impeachments. The removal of impeached officials is automatic upon conviction in the Senate.
The House of Representatives
Impeachment proceedings may be commenced by a member of the House of Representatives on their own initiative, either by presenting a listing of the charges under oath or by asking for referral to the appropriate committee. Non-members may trigger the impeachment process. For example, when the Judicial Conference of the United States suggests a federal judge be impeached, a charge of what actions constitute grounds for impeachment may come from a special prosecutor, the President, a state or territorial legislature, grand jury, or by petition.
The House debates the resolution and at the conclusion may consider the resolution as a whole or vote on each article of impeachment individually. A simple majority of those present and voting is required for each article or the resolution as a whole to pass. If the House votes to impeach, managers (typically referred to as House managers, with a lead House manager) are selected to present the case to the Senate. Recently, managers have been selected by resolution, while historically the House would occasionally elect the managers or pass a resolution allowing the appointment of managers at the discretion of the Speaker of the United States House of Representatives. These managers are roughly the equivalent of the prosecution/district attorney in a standard criminal trial.
Also, the House will adopt a resolution in order to notify the Senate of its action. After receiving the notice, the Senate will adopt an order notifying the House that it is ready to receive the managers. Then the House managers appear before the bar of the Senate and exhibit the articles of impeachment. After the reading of the charges, the managers return and make a verbal report to the House.
Senate
The proceedings unfold in the form of a trial, with each side having the right to call witnesses and perform cross-examinations. The House members, who are given the collective title of managers during the course of the trial, present the prosecution case. The impeached official has the right to mount a defense with his own attorneys as well. Senators must also take an oath or affirmation that they will perform their duties honestly and with due diligence (as opposed to the House of Lords in the Parliament of the United Kingdom, who vote upon their honor). After hearing the charges, the Senate usually deliberates in private. Conviction requires a two-thirds majority.
The Senate enters judgment on its decision, whether that is to convict or acquit. A copy of the judgment is filed with the Secretary of State. Upon conviction, the official is automatically removed from office and may also be barred from holding future office. The removed official is also liable to criminal prosecution. The President may not grant a pardon in the impeachment case, but may in any resulting criminal case.