The term dry wine probably is one of the most misused terms in the wine world by the average consumer. The truth is most of the wine that people consume on a daily basis are technically considered dry. The term "dry", in reference to fermentation technically means that there is less than 1% residual sugar in the wine. Most people generically use the term to describe a wine in where they taste no sweetness. A wine can be fermented "dry" and still taste sweet.
During fermentation the yeast consumes the sugar in the grape juice producing alcohol and COČ. The yeast will continue this process until all of the grape sugar has been used up at which point having no food source, the yeast cells die and become the lees. Wine is fermented to dryness because, among other things, leaving sugar in it would make it microbially unstable. Residual sugar in a wine which contains less than 16% alcohol creates a substrate for possible yeast growth, making the wine potentially unstable or liable to re-ferment in the bottle.
Most enologist consider primary fermentation as complete when residual sugars are in the range of 0.1-0.2%. Using the reducing sugar measurement, wine below 0.2% are generally considered stable in regard to the possibility of re-fermentation in the bottle. At the point most of the residual sugars present are pentoses which are unfermentable by saccharomyces yeast. However it is possible for microbial activity to occur even when sugar are at less than "dry" levels. Brettanomyces, a spoilage yeast, can utilize residual sugars well below the 0.2% level.
The standard range of residual sugar for table wine (wine you drink with dinner) is 0-3%. The following is a typical scale used to classify the sugar level in wine:
It is a misconception that dry wines cannot taste sweet. The sweetness of a wine on the palate is greatly influenced by the level of glycerol, alcohol, acidity, and tannins in the wine. Even the serving temperature of a wine can affect how sweet it tastes.
So, a "bone dry wine" with a residual sugar level of less than 0.2 % but one that is relatively high in alcohol and glycerol can actually taste somewhat sweet.
Sweet dessert wines can range from 1.2 % to 20 % residual sugar. In wines such as Sauternes where the grapes are picked at very high brix, fermentation stops at about 15 % alcohol as the alcohol begins to act as a preservative drawing all of the water out of the yeast cells. In the case of Port wines, alcohol is added to the wine to stop the yeast fermentation.
The post fermentation integration of grape sugar is not uncommon. Grape sugar masks the effect of acidity and tannin in a wine. Sometimes, grape sugar is added back after fermentation to disguise (hide) defects in lesser quality wines. That's why "jug" wines are usually fairly sweet - to hide flaws caused by using less expensive grapes.