There are as many types of wine as there are stars in the sky, or so it seems. From a bone dry Sagrantino to a very sweet White Port, there is a wine for everyone and every occasion. The sugar content, or the amount of sugar left over from fermentation is commonly expressed in grams per liter and will give you a general idea of how sweet a wine will taste. But while this may point you in the right direction, sometimes a dry wine will have a sweet taste to it. Why?
To answer the question, first we have to understand a bit about what makes up a wine. There is sweetness, acidity, tannin, fruitiness as well as body. The first, sweetness is how sweet it taste. But this is not simply how much sugar is in the wine. In wine there are many balance and counter balance, sweetness and acidity, for example. The acid content will affect how sweet a wine taste. A wine with a high sugar content may not taste very sweet if there is also a high acid content.
Another part of wine is fruitiness. Fruitiness and sweetness can seem very much the same, however while sweetness is mostly a function of taste, fruitiness is mostly aromatic. If a wine taste sweet, try plugging your nose and notice the difference. If it the sweet taste is mostly gone, then it was fruitiness and not sweetness. Sweetness will still taste sweet even in the absence of smell.
And while acid will make your mouth feel wet, tannin will make it feel dry. Tannin helps to balance out the sweetness of a wine, as well as help it last longer. The level of protein in your saliva has an effect on how your mouth reacts to the tannin content, and by extension, how sweet the wine taste.