Jackson's impeachment was done by the House, and was voted on in the Senate. He was impeached for not following the plans that Congress wanted for Reconstruction. He also fired one of his cabinet members, which there was an law against (this law was later stuck down as unconstitutional).
By having a bicameral legislation (2 parts Senate and the House), the balance of government is both for the larger states, and the smaller sates. It was also a check that one part of the legislative branch cannot easily remove a seating president.
AND YES THERE WERE 2 PRESIDENTS IMPEACHED- To be impeached means that the house brings charges against the president, so the Senate can vote for removal. That means there are 2 steps to remove a president, the first is impeachment.
