The earth is a giant magnet. Between the magnetic south pole of the geometric North Pole and the magnetic north pole of the geometric South Pole, there is a magnetic field and this is what is called geomagnetism.
For more details concerning the earth's magnetic field, please refer to the following explanation.(http://en.wikipedia.org/wiki/Earth's_magnetic_field)
A magnetic needle (a compass) points to magnetic north which is slightly different from the geometric north pole (true north). That said, depending on the location, deviations do occur when you look closely, as the "Declination" column in the chart below shows.
For how to correct declination, please refer to the Application Note on declination.
※ The value of the earth's magnetic field at cities and locations at any latitude/longitude not shown can also be calculated using the NGDC Geomagnetic Calculators.