The tip and top of your tongue are the areas that detect sweetness. One key aspect in the taste of different wines is the degree of sweetness or dryness. Simply put, a dry wine is a wine that is not sweet. Similar to tea or coffee with no added sweetener, dry wines have little or no natural sugar remaining after the fermentation process.
Sauvignon Blanc and white Bordeaux are good examples of dry wines, while Sauternes, late-harvest Riesling, Vouvray, White Zinfandel and Muscat Beaumes-de-Venise are generally sweet. A good contrast of sweet versus dry may be found in sparkling wines. A Brut Champagne will exhibit no sweetness at all, while a lush Asti Spumante will demonstrate ample sugar on the palate.
The perception of sweetness in wine can be deceptive. True sweetness is the result of residual sugar left in the wine from the fermentation process. There are other components in wine that can increase your impression of sweetness that are unrelated to residual sugar. Intense fruit flavors can be confused with sweet flavors, but a wine can be fruity without being sweet. Other components in wine such as tannin and acidity counterbalance the perception of sweetness. Many fine German Rieslings, for example, have such high acidity that they taste crisp and dry, even though they might contain higher levels of residual sugar than the average table wine. Lower levels of tannin and acidity can create, by their absence, a stronger impression of sweetness.