A Polar Star (or Pole Star) is an especially prominent star that is aligned (approximately) with the axis rotation of Earth. Its position is close to one of Earth's celestial poles, and lies directly overhead when observed from the North or South Pole.
The term Pole Star usually refers to Polaris, which is the current northern Pole Star. Also known as the North Star, it is the brightest star in the constellation of Usra Minor. Over time, however, a different star will take pole position.