Vin Doux Naturel
Literally this means "naturally sweet wine" in French. To be confusing it refers to wines that are not naturally sweet at all, but have had neutral grape spirits added to them to stop the fermentation process while there was still some unfermented sugars left. The process is called "fortification." Quite a few Muscat based wines are made this way in France, as well as the rare and incredible "Banyuls." Often abbreviated to VDN.
Port
There are 9 recommended top grape varieties for Port, with Bastardo being the most used, but not the highest quality. The best for quality are Touriga Nacional, Roriz and Barroca - the others have names with Tinta or Tinto in them - while 9 are recommend a huge number are legally allowed.
The grapes are grown on steep hillsides vineyards up the Duoro river. As the years go by, less of the prized hillside vineyards are used, eschewed in favor of the flat river side vineyards that can be maintained with tractors. The best grapes will always come from the steep hillsides.
The grapes are picked, often still by hand if the terraces are too steep. They are placed in a crusher (or for the tourist are still stomped on by foot) and left to ferment on the skins to get a dark color.
Once the fermentation is to the correct point (with some natural sugar left), the wine is is poured into huge containers called Cubas half filled with neutral grape alcohol (bad brandy). The alcohol kills all the yeast and fermentation ends.
After this, the wine is blended with other lots, depending on the type of Port it will end up being.
In the Spring the young Port is stored in containers called "pipes" and shipped by rail or river to the town of Oporto by the sea (actually by law, because of the fire risk, all Port must reside across the river from Oporto in a town called Vila Nova de Gaia).
If it was a good enough vintage, the best of the wine is bottled to be sold (after more 2 more years of barrel aging) as Vintage Port. The rest of the wine, and all of it in lesser years, is left in wooden casks to age even longer. Vintage Port rewards decades of bottle aging.
For Character Ports (house styles, with names like Sandeman's Warrior Port) the wine is blended again, to provide a consistent style, and then bottled, after a short period of wood aging (depending on the style). These wines do not improve with bottle age.
For Late Bottled Vintage Port (L.B.V.), the wood aging continues for at least 5 years, and then it is bottled. These wines do not improve with bottle age. They were created for restaurants to serve as a reasonably priced alternative to Vintage Port, and as they do not need decanting, to be easier to deal with.
Tawny Port is aged for many years in wooden barrels (the better Tawny Ports tell you the age of the youngest wine in the blend). As the Port ages, it throws sediment, just as Vintage Port does in the bottle. On a regular basis the wood Port (as all Ports still in a barrel are known) is drawn off the sediment and put into a new barrel (this is called racking). Each time the Port is racked, it looses some of its coloring agents, until the once dark red wine is "tawny" in color, and easy to see through. The best Tawny Ports rival Vintage Port in price. These wines do not improve with bottle age.
There are also Ruby and White Ports, but these are more popular in Europe where they are sold chilled as a cocktail. They are rare in the US. They are universally of low quality. Very inexpensive "Tawny" Ports are also found, mostly in Europe, that are a blend of Ruby and White Ports.
Sherry
A fortified wine made in the Sherry district in southern Spain around the city of Jerez de la Frontera. The wine is made primarily from the grape Palomino. The grapes are brought into the winery and pressed. The first pressed juice (that of the highest quality) is reserved to make the "fino" styles. The wine is vinified in the traditional manner, until dry.
Fino is placed into a partially filled barrel, and only fortified to about 15% alcohol so that the special yeast called "flor" can develop. The flor creates a barrier that protects the wine from oxygen.The result is a light colored wine that is usually rather dry. If the bodega (warehouse) is near the ocean town of Sanlucar de Barrameda the fino will be allowed to develop into the very dry Manzanilla style. Some claim to be able to taste the salt of the ocean breezes in this wine.
Amontillado starts out under flor, but is then exposed to oxygen to create a medium dark Sherry. It is soetimes sweetened to make a Cream Sherry.
Olorosos wine is placed in filled barrels and fortified to 18% alcohol to prevent spoilage or the accidental introduction of flor and then allowed to continue to age, and oxidize, developing a rich dark brown color and nutty flavor. The Olorosso style Sherry since it did not benefit from the introduction of the flor yeast is more likely to be sweetened heavily and end up as Cream Sherry.
After become the desired style of Sherry the Solera system comes into play. Six or more barrels are stacked up. Each of the barrels contains wine of different ages, in different proportions. Wine is drawn from the oldest barrel, and replaced with the next oldest, and so on. The theory is that in this way you "train" the younger wines. The final solera barrel may contain a fraction of wine that is fifty years old or even more. The Sherry that is brought to market is a blend of the wine from these barrels, and is never less than three years old.
Amontillado and Olorosos Sherry is sometimes sweetened just before bottling to determine its final style. The styles of sweetened Sherry, in increasing order of sugar added are: Dry, Pale Cream, Medium, Cream, Dulce / Sweet, Moscatel, Pedro Ximénez.
The sweetening agent is often concentrated grape juice from the Pedro Ximenez grape (PX is used on its own to make the sweetest style of Sherry). A final fortification is also performed before bottling to bring the final product up to 19% alcohol.