Directly from the link below, I found the best answer and illustrations that I could find..

"If you know the direction of true north from looking at the North Star, then you can point the N on the face of the compass in that direction (to the place on the horizon directly beneath the North Star - the red dot). Then you can see the angle between the black end of the needle (which points to magnetic north) and the N on the compass face!"
Source(s):
http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/magnetic.html

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True (geographic) north is defined by the earth's rotational axis and does not move with respect to the earth as a whole (although the axial tilt can change and tectonic plates can move across the poles).

Magnetic north is defined by the earth's magnetic field, which is generated by convection currents in the earth's outer core. A compass needle aligns itself with the local magnetic field lines, which converge at the magnetic poles. This magnetic field is not perfectly symmetrical, and the field lines do not follow straight lines between the poles the way lines of longitude do, so a compass needle may not point directly to the magnetic pole.

The actual angular difference between true north and magnetic north at a given location is called declination. The earth's magnetic field changes over long periods of time, and the magnetic declination at a given point changes over time as well.
Source(s):
http://www1.appstate.edu/~goodmanj/4401/notes/magnets/mnvsgn.html
http://en.wikipedia.org/wiki/Magnetic_declination

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