Geographic or Geodetic latitude : angle between equitorial plane and normal to the observers horizontal plane.
Geometric or Geocentric latitude : angle between line joing the observer to the geocentre of the earth. Geocentre is the point where earths axis crosses the equitorial plane.
The difference between them at poles and equator is 0 and maximum difference occurs at 45 deg lat where it is approx 11.4 deg., geographic higher than the geometric lat.
And from what I can recall, the precession rate varies as well, as the axis of earth's rotation is along geometric axis.
For a more detailed and very complicated explanation, visit
http://www2.ncdc.noaa.gov/docs/klm/html/i/app-i4.htm
Regards,
Divdby0