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True north is a constant and refers to the geographic North Pole. Magnetic north tends to shift and refers to the pole of the Earth's magnetic field. In mid 2002, true north and magnetic north were approximately 590 miles apart. This handy article from hiking expert Doug Latimer explains the poles of the Earth's magnetic field are different from its geographic poles. Maps are aligned along true north, so hikers have to make adjustments when navigating by compass. In navigation, the difference between true north and magnetic north is known as declination.
All U.S. Geological Survey maps print relevant declination information, and the maps are updated every five years to account for shift. Hikers traveling in Northern California, for instance, have to make declination adjustments of roughly 18 degrees. The Earth's magnetic field stems from its molten metallic core, much of which is iron. Iron is a fairly common element, since it can't be burned off during the fiery formation of stars. Iron is magnetic because its inner electron shells are slightly unstable. For much more information on this topic, Chris Goulet hosts a detailed Magnetic Declination FAQ.
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