Well, from my Map & COmpass courses I took many Year's ago:
TRUE NORTH The definitive North Pole, the point at which the globe rotates. True north is a navigational term referring to the direction of the North Pole relative to the navigator's position. Its concept was first discovered and noted by the Chinese polymath Shen Kuo in the 11th century.
Grid North Grid north is a navigational term referring to the direction northwards along the grid lines of a map projection. It is contrasted with true north (the direction of the North Pole) and magnetic north (the direction of the Magnetic North Pole). Many topographic maps, including those of the United States Geological Survey and the Ordnance Survey, indicate the difference between grid north, true north, and magnetic north.
MAgnetic North The magnetic declination (also known as grid magnetic angle in military circles) at any point on the Earth is the angle between the local magnetic field -- the direction the north end of a compass points -- and true north. The declination is positive when the magnetic north is east of true north. The term magnetic variation is equivalent, and is more often used in aeronautical and other forms of navigation. Isogonic lines are where the declination has the same value, and the lines where the declination is zero are called agonic lines