What is the difference between a sweet wine and a fortified wine?
A sweet wine is one in which there is a degree of residual sugar after the fermentation of the wine. This can be achieved in many different ways. Either the sugar in the grape is concentrated by raisining the grape or by Botrytis cinerea or by super-ripening it, so that there is more sugar than can be fermented. Or sugar is added to a wine, or fermentation is stopped before all the sugar has turned to alcohol. Examples range from Sauternes to Tokaji, Vin Santo, Coteaux du Layon, Cordon Cut Riesling. Fortified wines are just that. The wine has grape spirit added to it before fermentation has finished to stop the fermentation whilst still leaving the wine sweet and also increasing the overall alcohol content of the wine.
