Sweet wines are made from grapes that have concentrated sugars, this means that the grapes are left on the vine to ripen to their fullest.  After the grapes ripen they soon if not picked start to concentrate the sugars within and the result is a process known as Noble Rot or to give it its scientific term (Botrytis cinerea). The concentration process can occur in different ways depending where the grapes are grown and the sweet wine making tradition of the Country or wine region.  In Italy some sweet wines are made by laying the grape bunches out on straw mats until the juices have concreted enough to be made into wine.  The famous sweet wines of sauternes in South West France grapes are left on the vine until the juices concentrate before they are picked.  In Germany the famous icewine is made by leaving the grapes through the winter and picking the grapes at night when they are frozen, then crushing the grapes and discarding the ice leaving the concreated sweet juice. Sweet wines tend to be more expensive due to their labour intensive production,  but are the most amazing wines you can taste.