Geographic Tongue is a condition in which the tongue shows light markings which have been likened to a geographic map.
It's a very common condition, estimated to affect around 2 - 14% of the entire population at any given time.
It's thought to be completely benign. It can appear at any age (though children appear to be more prone to it) and can spontaneously disappear.
There are currently no explanations about its cause, or its clinical significance (if any).
See : Wikipedia (with photos).