I would like to know what is impeachment and witch president in USA have been impeached ?

Impeachment is a constitutional procedure for removing a President from office. He must be indicted by a majority of the House of Representatives before being tried before the Senate. In all of U.S. history only Andrew Johnson and Bill Clinton have been indicted and tried. Neither was convicted, and both served out their terms although Johnson, whose, "crime," involved resisting the harsh measures Congress wished to impose on the beaten south, fared much worse in his subsequent career than Clinton, who had only perjured himself before a Federal Grand Jury.