When it really comes down to it, the sweetness of a wine is variable. When we read the sweetness levels on bottles of wine, we must not take them to literally. The way we perceive the sweetness depends on the food we are eating or wine we previously drank (which is why, you are always supposed to drink wine in order from the driest to the sweetest). On top of this, other factors like acidity and tannins will have an effect on our perception of a wine’s sweetness. Typically a wine with a high acidity will be perceived as dry.
The sweetness of a wine can be enhanced in a number of ways:
During fermentation, yeast interacts with the sugars in the grape must creating ethanol (ie. alcohol.) The longer the fermentation process, the more sugar is converted into alcohol, and so the more concentrated with alcohol a wine is and the less sugar it has. The higher levels of alcohol in the wine, the less sweet tends to be. To maintain a certain level of sweetness in the wine the fermentation process must be stopped, so there is a certain amount of sugar left over. The sugar left over is known as residual sugar, and it is what makes the wine we drink sweet.
Another way to produce a sweet wine has to do with the time of harvesting. Grapes harvested late in the season tend to be much sweeter. In their youth, grapes are quite acidity and have little sugar. Once the fermentation process has come to an end, the grapes remain high in acidity but have little to no residual sugar, creating a dry wine.
Another method, though seemingly counterintuitive to the last, involves picking the grapes in their youth, and then drying them in the sun. This ensures that the wine is both acidic (due to its youth) and sweet (due to drying them in the sun.)
An alternative method is one that is highly regulated around the world. Known as chaptalization, this process involves adding sugar to the grape must prior to fermentation. This process may be used to both make the wine sweeter and to make it more alcoholic.
A final sweetening method involves allowing mature grapes to be frozen before harvesting. Once picked, the grapes are pressed, causing the sugary syrup to bleed out of the grape while the water stays solid. This produces the popularly sweet- Icewine or Eiswein.