The tongue is capable of tasting four distinct types of flavors: sugars, salts, sours, and bitters.
The front tip of the tongue is most sensitive to sweet flavors.
On either side of the tip are two slightly less sensitive areas that can detect salts.
Along both sides of the tongue are sensors used for tasting acids and other sours.
In the middle, at the back of the tongue, is an area sensitive to bitter flavors.
The geography and different sensitivities of the tongue give good reason to use a properly shaped glass when tasting wines.