What's the difference between a turnover and a turnover on downs in the sport of American Football?

A turnover is when the offensive team, the team with possession of the football, loses possession of the football during play. This generally occurs in two ways: the defense intercepting a pass or recovering a fumble. A turnover on downs occurs only on the fourth and last down of a set of downs; the offense attempts to gain the yardage required to be given a new set of downs but fails to do so by means other than the defense's takeaway, such as an incomplete pass, or the ball carrier being tackled short of the line to gain.