Dessert wines are sweet wines usually served with dessert.
A dessert wine is considered to be any sweet wine drunk with a meal, as opposed to the white fortified wines drunk before the meal, and the red fortified wines drunk after it. Thus, most fortified wines are compared as distinct from dessert wines.
In the United States, oppositely, a dessert wine is legally defined as any wine over 14% alcohol by volume, which includes all fortified wines. This dates back to a time when the US wine industry only made dessert wines by fortification, but such a classification is outdated now that modern yeast and viticulture can produce dry wines over 15% without fortification.
German dessert wines can contain half that amount of alcohol.
Winemakers want to produce a dessert wine containing high levels of both sugar and alcohol, yet the alcohol is made from sugar. There are many ways to increase sugar levels in the final wine:
Grow grapes so that they naturally have sugar to spare http://1000-facts-about-wine.com for both sweetness and alcohol.
Add alcohol before all the sugar is fermented, this is called fortification, or 'mutage'.
Remove water to concentrate the sugar.
Honey was added to wine in early Roman times, for sweetness and to increase the final strength of the wine. Today sugar is usually added to boost the alcohol levels of flabby, unripe wines rather than for sweetness, although a degree of chaptalization is permitted in the wines of many countries. German wines must declare whether they are 'natural' or not; presently, chaptalization is banned from the top tiers of German wines.
The main rule in serving is that the wine should be sweeter than the food it is served with - a ripe peach has been described as the ideal partner for many dessert wines, whereas it makes sense not to drink wine at all with many chocolate-based dishes. Red dessert wines like Recioto della Valpolicella and fortified wines like the vin doux naturel Muscats http://1000-facts-about-wine.com are the best matches for such difficult-to-pair desserts.