Treason and bribery are fairly easy to define and if a U.S. president or other federal official is accused of one of those crimes, the impeachment proceedings are on reasonably solid ground. But what about "high crimes and misdemeanors"?
It turns out that when the founders first drafted the standards for impeachment (Article 2, Section 4 of the constitution), they only listed treason and bribery as the actions that could trigger the process. But a framer by the name of George Mason pointed out that a federal official could try to "subvert the Constitution" without committing treason or deploying bribery. He tried shoe-horning in the term "maladministration," but James Madison found the word unclear.
So Mason took it out and shoved in "high crimes and misdemeanors," failing completely to clarify the matter [source: Stewart]. By that time, we have to assume, the framers had reached peak revision-fatigue and just left those four words in as place-savers until better vocabulary could be wrought. This eventuality failed to materialize.
The problem has vexed scholars, lawyers, officials and impeachment procedures ever since. Taking out "maladministration," seems to indicate that simply being bad at the job of commander in chief isn't enough to get you fired. The "crimes" part of "high crimes," would seem to suggest that a president has to commit a crime while in office, but some have argued that saying "high crimes and misdemeanors" means they're of a different order and indicate a betrayal or subversion of the political office.
In other words, for some people it's all about the crimes and for others it's all about the politics. This confusion is of a piece with the dual, or hybrid nature of impeachment, a process which looks, sounds, talks and walks like a judicial proceeding, but has a purely political result — removal from office. In other words, impeaching a president doesn't send him (still him, so far) to jail.
An impeachment procedure starts life in the House of Representatives where there's a vote to impeach on at least one of the articles (treason, bribery or the infamous high crimes and misdemeanors). If the vote gets a majority, the president is officially impeached, which amounts to the political equivalent of a criminal indictment. To be impeached, in other words, is to be officially accused of a wrongdoing worthy of removal from office [source: Savage]. But if you're an impeached president, the fat lady hasn't sung yet.
Next, the matter moves to the Senate. There, a group of representatives from the House comes along to perform duties equivalent to criminal prosecutors. But since this isn't a prosecution, they're not called prosecutors, they're called — for reasons unknown — the "managers." The managers argue their case before the Senate, which stands in here for a jury. At the end of the impeachment trial, the senators vote.
To find a president guilty of the charges laid, fully two-thirds of the Senate must vote "guilty" [source: Savage]. If they do, the POTUS is potted. He's goners. But if the vote is anything less than two-thirds against he gets to stay. This is why a president can be impeached by the House but acquitted by the Senate and thus remain in office.