On February 24, 1868, Andrew Johnson became the first president to be impeached. He would not be the last.
Impeachment is a formal process brought about by both houses in Congress that accuses the president of breaking the law. If found guilty by both houses in Congress the president may be removed from office, sued, and/or sent to jail.
Only two presidents have been impeached. Presidents Andrew Johnson and Bill Clinton. Both presidents were impeached by the U.S. House of Representatives, but acquitted by the Senate. Andrew Johnson was impeached for firing the Secretary of War, Edwin M. Stanton, which was a violation of the Tenure of Office Act. Bill Clinton was impeached for lying about having a relationship with an intern. Both presidents were able to complete the term in office.
Richard Nixon quit before he could be impeached.