True north is the direction along the earth's surface towards the geographic North Pole.
True north usually differs from magnetic north (the direction of the magnetic north pole) and grid north (the direction northwards along the grid lines of a map projection).
The direction of true north is marked in the skies by the north celestial pole. For most practical purposes, this is the position of Polaris. However, due to the precession of the Earth's axis, true north rotates in an arc that takes approximately 25,000 years to complete. In 2102 Polaris will make its closest approach to the celestial north pole. 5,000 years ago, the closest star to the celestial north pole was Thuban.
On maps published by the United States Geological Survey, and the U.S. military, true north is marked with a line terminating in a five-pointed star. The east and west edges of the USGS topographic quadrangle maps of the United States are meridians of longitude, thus indicating true north (so they're not exactly parallel). Maps issued by the United Kingdom Ordnance Survey contain a diagram showing the difference between true north, grid north and magnetic north at a point on the sheet; the edges of the map are likely to follow grid directions rather than true, and the map will thus be truly rectangular / square.