Port is a "fortified wine." Brandy is added to the wine to stop fermentation before the yeasts eat all the grape sugar, thus yielding a sweeter wine and higher alcohol content.
True Port comes from Portugal (the Duouro region, to be exact). But since winemakers in other countries have taken to producing "Port," Duouro Port makers have started to call their Port, "Porto," or "Oporto" (from the city in Duouro).
There are two main categories of Port: Vintage Port and Wood Port.
Wineries will decide ("declared year") that the harvest in a some particular year (or "vintage") is worthy of producing this port, which is aged for two years in wood from grapes of that harvest year only. It will also continue to mature once bottled. Not only are not all years declared to be vintage years, but not all wineries may decide within a particular year that their wine is a vintage year, and even in a declared year (which may occur two or three times in a decade) perhaps only 10% of the grapes will go into vintage port (with the balance going to wood ports). So in most years there just is no vintage Port at all!
Vintage Ports get much better with age. Generally don't drink them before they've aged fifteen years. Some can keep getting better for a long time after that--even one-hundred years. Like most good wine, a vintage port shouldn't be left around undrunk once opened.
Single-Quinta Vintage Port.
Single-Quinta Vintage Port is true vintage Port--wine from one harvest year bottled unblended after two years in cask. When a shipper "declares a vintage," the vintage Port from that year usually comes from wines produced by grapes from various vineyards (quintas). It is said that no one vineyard has all the characteristics to make the best vintage Port--it needs to be blended with other vineyards to be the most complete and complex wine. However, sometimes a producer's single best vineyard will yield grapes fine enough to warrant bottling on their own, while the rest of the vineyards that would normally contribute to a vintage Port weren't as successful. The producer may then choose to vinify this wine from that single vineyard, or "quinta". This is called "single-quinta vintage Port" and the quinta name will appear on the label. So, whereas a Port labelled "Graham's 1991 Vintage Porto" is a vintage Port from a declared year, "Graham's Malvedos 1988 Vintage Porto" is a single-quinta vintage Port from the Quinta dos Malvedos, the best vineyard that Graham's owns.
The one exception to this nomenclature is the Quinta do Noval, which is actually a producer, not a single quinta. (Noval's best vineyard is called Nacional, and its single-quinta Port is the rarest, most expensive, and reportedly best of all.)
There are three sub-categories of Wood Port, based on color: Ruby Port, Tawny Port and White Port.
Ruby Port. A dark red, somewhat sweet "full-bodied" wine which has probably been aged in wood for several years.
Tawny Port. Not such a deep color, it is a "smoother," less sweet wine which may have been aged in wood for 20 years. The difference between tawny Port and ruby Port is simply the amount of time that the wine spends in the wood cask before it is blended and bottled. As the wine ages, the ruby-red color of the young wine becomes paler and browner. Top tawny Ports from the best producers are just as complex and fine (and expensive) as vintage Port, though they will have a different character. (If you find something labeled tawny Port which seems inexpensive--or shall we say, "cheap?," you may have found something produced by blending "tawny" Port with "white" Port. Needless to say, you'll tell the difference and Port connoisseurs will tell you that they aren't worthy of the name "Port" at all.)
White Port. A sweet white wine made from white grapes grown in the Oporto region of Portugal. As with red Port, fermentation is stopped by adding brandy to the partly fermented wine. Not really like the other (red) Ports, which are usually drunk after a meal, this is usually drunk before a meal.
Wood Ports will not get any better by cellaring, so you can drink them as you buy.
As you age your good Port it is going to "throw off" a good amount of sediment which is going to end up in your glass if you don't decant. So, get into the habit of decanting. Unless you like to eat sediment, of course.
For Further Information on Port
I have no knowledge of, but repeat posted information that there is a a quarterly newsletter called Re: Port. P.O. Box 981, Cherry Hill, New Jersey 09003. Said to list availability and best retail prices for vintage port in the U.S. Apparently a sample copy is available.
[For the Future: Expanded discussion of Port. I've got forty pages of notes!]