What is the difference between magnetic north and true north?

True North is the axis of rotation of the earth.  Magnetic north is the northern magnetic pole.  The magnetic poles do not lie on the axis of rotation so there is some difference.  The angle of deviation depends on where you are.  If you are exactly true south of the magnetic N pole, there is no deviation.  The max devation is if you are 90 degrees of longitude away.  If you want to be even more accurate, you need to take into account local deviations in earth's magnetic field (the iron content of the bedrock has an influence).

If you check out charts for aviation or for marine navigation, these charts are specific to particular regions and you can get them specifically for a particular air traffic control area, like Boston or a particular area of ocean or large lakes.  These charts list the deviation between True N and Magnetic N.

true north is true to what it was years ago. mag north is what the compass points you to now