The Discerning Photographer has a tutorial on how to use Phtooshop to correct perspective, i.e. get leaning verticals straight again.
This is more apparent when photographing buildings either with a wide angle lens or when you tilt your camera upwards to try to include the roof of the building in your shot. The result is leaning verticals where the top of the building is distorted, appearing too small, as if it’s falling backwards.
Read the tutorial at: The Discerning Photographer.