Fortified wines are made when brandy is added to the must to cut the fermentation process. Yeasts do not finish their work of transforming sugar into alcohol and the final wine has a high level of sweetness.
Most fortified wines are higher in alcohol content (about 17-20% and have a longer shelf life). Their styles vary by their content of sugar and alcohol, but also by their time of aging.
The most famous examples of this category are the Portuguese Porto and Madeira, the Italian wine Marsala and the Spanish Jerez.
Within the category of fortified wines, we must also include Vin Doux Naturel (natural sweet wines). These are made using a style similar to Porto. The term comes from France, but this classification could be used to describe other wines.
For example, in the French region Languedoc-Roussillon are produced from the strains Grenache and Muscat.