Geographic tongue is also known benign migratory glossitis of the tongue.
What is means is the little papillae on a tongue with this condition are almost constantly inflamed somewhere. This creates odd patterns on the tongue with grey and red colors. The colors are made because the red areas are actually completely devoid of any papillae (and smooth-looking) and the greyish areas are where the papillae are overcrowding!
What's really neat about this though are that the different markings on the tongue change constantly! Like I had a patch on one side of my tongue maneuver to the other side and split into 2 mini-patches in about a day.
While studies vary, it has been figured that at any one time between 0.1 percent and 14.3 percent of all people have this interesting feature!
Women are generally affected 3 times more often than males.
Some foods can trigger and make the condition more noticeable (like the grey patches will get really white or something).
Doctors are still unsure what exactly causes it but it is known to run in families.
Tongues with this have normal taste and dexterity; however, they do loose some sensitivity to touch.
Interestingly enough it seems to be more common in people who are sensitive to the environment (like allergies or asthma).