A pole star is a star that is located almost due north or due south, which makes it useful for navigation. Polaris is the pole star of the Northern Hemisphere. There is no pole star for the Southern Hemisphere; the constellation Crux, however, is around the south celestial pole.
A pole star is a visible star that is approximately aligned with the Earth's axis of rotation; that is, a star that lies in the direction pointed to by one of Earth's poles. There are potentially both north and south pole stars, but whether there is either depends on the current stellar configuration. The term the pole star usually refers to the star Polaris (colloquially referred-to as the "north star") which is the current northern pole star.