While your nose can detect thousands of smells, your taste buds can sense only sour (acidic), salty, sweet, and bitter tastes, of which the latter is usually considered a faulty characteristic in a wine. The main culprit behind bitter wine is an excess of tannin, which comes from the grape's skins, seeds, and stems (hence winemakers are careful not to crush the seeds). A hint of bitterness is common in some wines, such as Chianti, but the bitterness should never dominate the flavor or aftertaste. FYI, your bitter-sensing taste buds are on the back of your tongue.