Tawny Port refers to
port that has been aged in
wood for a long time (several years). Generally, ports from several different years are aged in this fashion, and then blended to create a port that is of the
average age of its constituents. This tends to create a product of consistent taste.
Production
Wines from the non-vintage port years go into tawny ports. These wines are fermented in the usual fashion of port and then placed in oak barrels and allowed to age in a cellar. Most tawny ports you will find are aged in the wood 10, 20, 30 or 40 years. During this aging process a significant amount of the essence of the wine evaporates through the wood (as in the making of whisky or cognac), leaving behind a much more complex product that has a characteristic "nutty" flavor. The wine also changes color, going from the very dark purplish-black of traditional port wine to a lighter, dark red/amber/brown color.
The longer the wine is allowed to age, the more concentrated and less wine-like the final product. Whereas vintage ports are very complex and concentrated, but still very much a wine due to their bottle aging, tawny ports have a much different flavor and should be appreciated as such.
Variants
Most port houses make tawny ports. Port you find on the shelf called simply tawny port are often a cheap mixture of red and white ports and are often seriously crappy, so avoid these. Good tawny ports generally have "X year old port" on the front of the bottle (the bottle may not say tawny, or it says so in fine print on the reverse side, but it IS a tawny port). The number X is usually either 10, 20, 30 or 40, describing the average age of the tawny port. The dollar amount you will pay for the port goes up accordingly, but the older tawnies are worth it, trust me.
Ports labeled as colheita ports are tawny ports in which no blending has been done. The product in the bottle is from a single quinta, and often a year appears on the bottle. This is the top-of-the-line tawny port.
Summary
Tawny ports are created in a different style than are vintage ports, but their unique style is still very awesome and should not go unappreciated. Get a good bottle if you see one and enjoy it with some bread and cheese!
Some notes:
In Alabama, since liquor is sold by the state, stores only carry liquor on the state's "approved" list. Since port is 20% alcohol, it is classified as hooch and is sold by the state. Consequently, the selection of port in this state is very poor. I have only seen the 10 and 20 year Fonseca Tawny Port for sale. There is no vintage port for sale (I have to buy this in quantity when I go on trips). sigh.