Wine is a liquid right? So why are people always asking if their wine is “dry”?
When it comes to alcohol, dry just means “not sweet”. Different wine regions have different ways of designating sweetness- in Champagne “Brut” means dry, in Germany “Trocken” means dry, in France and Italy they basically just expect you to know enough not to ask. But it all comes back to the same basic concept- residual sugar.
Grapes are fruit, which means they contain sugar. The amount of sugar in the grape depends on the varietal (Pinot Noir, Chardonnay, Gewürztraminer etc), and how ripe it is when picked. When yeast reacts with the crushed grape juice, it begins to eat the sugars and convert them to alcohol. If left alone, the yeast will keep eating sugar until the alcohol level in the juice becomes toxic and it dies (yeast cells aren’t very smart). If there is still sugar leftover at that point, it remains unconverted and is called residual sugar.
Yeast dies at around 14-18% alcohol by volume, which is a lot of booze (most wines are around 11-13%). So most of the time, winemakers will stop the fermentation process before the yeast has a chance to overdose, or they’ll only add enough to convert the just right amount of sugar in the first place. This way they get a nice, dry wine with a balanced alcohol content so you aren't wasted after one glass.
Of course there are your dry and off-dry (slightly sweet) wines for sipping and pairing with dinner, and then there are true dessert wines; wines designed to be sweet and syrupy. Those wines are made differently, but rely on the same concept: that grapes basically contain sugar and water. Each of the four methods below essentially achieves the same goal; reducing the amount of water or increasing the amount of sugar in a grape before it is turned into wine.
Late Harvest- Grapes are allowed to ripen on the vine past the point where they would normally be picked. This results in super ripe, super sweet grapes, which produce sweeter wine.
Noble Rot- A fungus called Botrytis is allowed to grow on the grapes, which sucks water from them to feed itself. As a result the grapes have a higher concentration of sugar inside when harvested. Botrytized wines are difficult to make, and highly prized.
Raisinated- Picked grapes are allowed to dry slightly in the sun before being pressed, again resulting in a higher concentration of sugar.
Ice Wine- Grapes are allowed to freeze on their vines and are pressed while still frozen. The sugars don't freeze, but the water does, so the resulting juice is very sweet. Ice Wine is very rare and expensive.
Of course, if you want to achieve sweetness you could always just take a dry wine and stir in some sugar, and some commercial wineries do exactly that. It’s cheaper, easier and altogether lamer than the aforementioned methods.