Andrew Johnson, 17th President of the United States, stands out from other presidents for his humble origins- he was born in a log cabin to illiterate parents. Until he was 17, he had not mastered the basics of math, grammar, or reading.
Lincoln, in his attempt to show his support for the unity of the country, ran under the National Union Party rather than the Republican Party for his second term. He also chose a Southerner as his running mate- Andrew Johnson.
Naturally, when Lincoln was assassinated, Johnson rose to power. He oversaw the reconstruction of the nation following the Civil War, and he chose a decidedly lenient and Southern-sympathetic approach. He pardoned those who took oaths of allegiance, though he required wealthy men to get special Presidential pardons.
He allowed many Confederate generals to stay in power and did very little about "black codes," regulations severely restricting the freedom of former slaves. Radical Republicans in office adamantly tried to combat Johnson's easygoing Reconstruction policy. Despite Johnson's veto, Congress managed to override him and pass the Civil Rights Act of 1866 as well as the 14th amendment in the defense of freed slaves.
The Congress, fully opposed to the President, eventually attempted to impose several restrictions on him, including the Tenure of Office Act. They claimed that Johnson had violated this act with the dismissal of Secretary Stanton, and attempted to impeach him. Johnson was acquitted by just one vote. His presidency ended in 1869, leaving a legacy that would cause him to be consistently ranked amongst the worst US presidents (in sharp contrast to his predecessor, Abraham Lincoln).
Sources: Miller Center, whitehouse.gov, Wikipedia