The North Star is that star which is best suited for navigation northwards. A candidate must be visible and circumpolar to the north celestial pole.
The North Star has been historically used by explorers to determine their latitude. At any point north of the Equator the angle from the horizon to the North Star (its altitude) is the same as the latitude from which that angle was taken. For example, the angle to the North Star for a person at 30° lattitude will be about 30°.
Polaris is the current North Star, but has a visual magnitude of only 1.97. On the other hand, in 3000 BC the faint star Thuban in the constellation Draco was the North Star; and at magnitude 3.67 it is five times fainter than Polaris. The bright star Vega will be the North Star by 14,000 AD.
Currently, there is no South Star as useful as Polaris; the faint star σ Octantis is closest to the south celestial pole. However, the constellation Crux, the Southern Cross, points towards the south pole.