http://www.ordnancesurvey.co.uk/oswebsite/aboutus/reports/misc/north.htmlOrdnance Survey wrote:Grid North: the direction of a grid line which is parallel to the central meridian on the National Grid.
True North: the direction of a meridian of longitude which converges on the North Pole.
Magnetic North: the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate and currently is west of Grid North in Great Britain.
Basically Magnetic North isn't at the North Pole, True North is and Grid North allows for representing the spherical Earth on a flat pice of paper without bending the lines (I think).
But since orienteering maps use Magnetic North it isn't a problem, except very occasionally