About the maps
These maps of historical magnetic variation (or declination) are derived from a small flash animation on the USGS geomagnetism web site. That site has since been updated with an animated GIF. I've prepared a larger animated GIF with ten-year intervals based on the original animation.
USGS site with small animated
Large animated GIF (10-year intervals)
Individual frames and viewer (10-year intervals)
The original maps include this credit line: "Model by A. Jackson, A.R.T. Jonkers, M.R. Walker, Phil. Trans. R. Soc. London A (2000). 358, 957-990." The primary web site for geomagnetic information from the USGS is http://geomag.usgs.gov.
What is magnetic declination?
A magnetic compass aligns itself with the local magnetic field on the Earth's surface. Usually, the needle points approximately towards the Earth's geographic north pole, but a compass rarely indicates true north exactly. The difference between the direction the needle points and true north is called magnetic declination or variation. Both ternms, declination and variation, are in common use (though neither one is particularly descriptive). Variation is the normal term for mariners and navigators at sea. Declination is prefered by geologists and land surveyors and most everyone else stuck on shore.
The maps on these pages display lines of contant magnetic declination/variation. For example, at all points along a line marked 10 degrees, a magnetic compass needle will point ten degrees west of true north. The Earth's magnetic field is changing in a complicated and largely unpredictable fashion; the magnetic declination changes significantly over the course of a few decades. There are more details available on the USGS web site.