Without sugar, there is no wine.
Sugar is a critical ingredient in fermentation. It naturally occurs in grapes as they ripen. During the winemaking process, the sugar in the grape juice is broken down and converted by yeast into alcohol and carbon dioxide. The fermentation slows or stops when the grape sugar fully converts into alcohol, meaning no or very little sugar remains in the wine.
While the addition of sugar is not legal in still winemaking, methods like Chaptalization allow the same. Here, unripe grapes are fermented with cane or beet sugar to boost the alcohol content in the final wine. However, only a handful of wine regions legalizes this process.
Winemakers worldwide intervene during the fermentation process to control the amount of sugar in the wine, a.k.a Residual Sugar or just RS and achieve the desired style. The more the RS in the wine, the sweeter it will taste, and the lower the RS, the drier the wine will be.
THE SWEETNESS SPECTRUM
In simple terms, sweetness is an indicator of how much RS a wine contains, measured in grams per liter (g/L). 'Dry' and 'Sweet' are the two opposite ends of the spectrum, and within these, there is an entire range. It is worthwhile to note that most red and white wines fall in the dry wine category.
Typically, wine professionals use the following terms to describe the sweetness level in still wines:
Bone-Dry: less than 1 g/L RS
Dry: 1 - 10 g/L RS
Off-Dry: 10 - 35 g/L RS
Medium Sweet: 35 - 120 g/L RS
Sweet: 120+ g/L RS
Look for these terms or tasting notes on the wine labels to understand how sweet the wine will be. Or, dig into the technical sheets that are available on the brand/company websites.
Unlike still wines, a certain amount of sweetness (concentrated grape must or sugar), called dosage, is added to sparkling wines before corking the bottle. As sparkling wines tend to be very acidic, the added sweetness helps to reduce the sourness of the wine and brings a balance. Wine producers indicate the level of sweetness (see below) in sparkling wine or Champagne on the labels.
Brut Nature: 0 - 3 g/L
Extra Brut: 0 - 6 g/L
Brut: 0 -12 g/L