Saying a wine is "dry" may be confusing because it is often mistaken for the dryness sensation left in the mouth after a sip. That sensation is actually caused by the level and roundness of tannins in the wine.
On the contrary, a dry wine does have no or little residual sugar, and it is literally the opposite of sweet wine.
Several factors impact directly on the level of sweetness of a wine. Most importantly, the grape varietal (some varietals develop more sugar than others), the ripeness at the moment of harvesting (the riper the grape, the more sugar it has), the terroir and the weather (the temperature and sun exposure aid to the ripeness) and fermentation methods.
How are dry wines are made?
To eliminate all hints of residual sugar, most wines are fermented to dryness. During the fermentation the yeast feeds on the sugar of the must, producing secondary flavours and alcohol in return. When the sugar is finished, the fermentation stops, reaching alcohol levels up to 12-14% ABV.
Most - if not all - of the wines you normally drink are dry. Any Syrah, Chardonnay, Cabernet, Sauvignon Blanc or any blend... those are wonderful examples of dry wines.
How are sweet wines are made?
We have already established that sweet wines contain unfermented residual sugars. They can be made in several ways, impacting the flavours and style of the wine:
Adjusting the sugar levels of a fully fermented dry wine by adding unfermented grape must. Some semi-dry German wines are made through this technique . Adding grape spirit during the fermentation (fortification), stopping the process. Wines made this way have high levels of alcohol. Port and sherry are made this way.
Chilling the wine below 5 degrees celsius. Under that temperature, fermentation cannot take place. Kabinett Riesling or Moscato d'Asti are great examples, sweet and with low alcohol levels.
Grapes affected by noble rot concentrate has high levels of sugars, that will not be consumed during fermentation. Sauternes, Tokaji, or the best sweet Rieslings are produced through this technique.
Allowing fully ripe grapes to partially dry on the vine (also known as passerillage). Wines made through this technique will show "late harvest" on the label.
Letting grapes dry after picking. This technique has similar results to passerillage: dehydration concentrates the sugar in the grapes. This technique is used in Italian passito wines, such as Reciotto della Valpolicella DOCG.
Allowing the grapes to freeze in the vine. This is how the Canadian Icewine or the German Eiswijn are made.
Are dry wines better than sweet wines? It is a matter of taste. Dry wines pair well with savoury dishes. Sweet wines are great to pair with dessert, yet also create interesting contrasts when paired with savoury dishes. The world of wine is exciting and full of variation and richness. The most important thing is that you learn what you like, dismiss what you don't, and keep enjoying the discovery process!