| Marsala is a type of fortified wine (see also sherry, port and madeira) that is produced near the port of Marsala in western Sicily.
In the late 18th century, an English businessman named John Woodhouse discovered that this region produced a large amount of sweet wine, and had the bright idea of introducing the methods used in Spain and Portugal for making the dessert wines that were popular in Britain at the time.
Woodhouse opened a wine-importing business in 1773 and was so successful that in 1813 a Mr. Ingram began a competing firm. In 1832 the House of Florio, an Italian company, joined the fray, and in 1929 Florio bought out both Woodhouse and Ingram. Today, Florio is still the main producer of Marsala.
The base of Marsala is a white wine made from local Sicilian grapes, which already contains 15-17% alcohol. They then add a sweet grape juice called sifone which has been prevented from fermenting by the addition of about 20% alcohol, and a kind of grape juice syrup called cotto, which is slightly caramelized and "gives color, bitterness and a trace of almond flavor" (The Cook's Encyclopedia).
The finished wine may have more alcohol added if necessary, and then, according to TCE, it is "cleared--usually with ox blood," horrifying as that may sound. It is then aged for at least two years in a wooden cask, and ends up at about 17-20% alcohol and 5-12% sugar. Marsala is usually sold in dry and sweet varieties, and may be served as an aperitif or used in cooking (it is an essential ingredient in zabaglione). It will keep indefinitely if the bottle is corked.
Sources
Jonathan Bartlett, The Cook's Dictionary and Culinary Reference, 1996.
Tom Stobart, The Cook's Encyclopedia, 1980
L. Patrick Coyle, The World Encyclopedia of Food, 1982 | 
