There's a quotation from the (in)famous Samuel Johnson:

Claret is the liquor for boys; port, for men; but he who aspires to be a hero [smiling] must drink brandy.

It's a very English quotation; Claret (pronounced "klar-it") is a peculiarly English name for a Bourdeux style wine. Port, particularly in Johnson's era, was an English masculine dinner ritual, followed, a hundred years later, by the gentlemen smoking cigars with their Port.

Port is a fortified wine. It is usually red, but it may also be white. It is "fortified" by the addition of distilled "grape spirits," essentially grape brandy, which stops the fermentation process before all the sugar has been converted. It is then allowed to age; Port is a wine that very much improves with age. Port tends to be a sweet wine, though it comes in a range of sweet to dry. The addition of the grape spirits, or aguardente in Portugese means that Port is typically higher in alcohol than most wines; 19% to 20% abv. Port was initially fortified this way in the eighteenth century, to insure that Portuguese wine would survive the voyage to England. You will, to this day, see the names of those original shipping and export companies on Ports, like Sandeman and Taylor.

Port is most commonly served after dining, as a dessert wine, and it is frequently accompanied by cheese. Stilton is often the cheese of choice to accompany Port. The English, as a side effect of the Royal Navy and the export of Port during the war with France, has made the consumption of Port both an art form and a ritual. Today, White or tawny ports are increasingly common as a before dinner apéritif; white Ports are made from white grapes; tawny Ports achieve their lighter color because of the aging process.

Technically, only Port that comes from Portugal's Douro valley in the north is Port, at least in Europe and to purists. The name Port is derived from the name of the Port at the mouth of the river that forms Portugal's Douro Valle, Oporto. There are a variety of Port types, and true Port lovers are rather insistent about the variety they prefer. In the U.S. (and Australia and South Africa) you will find a Port-style wines, particularly from California. At this point, I want to make it very clear that the differences between Portuguese Port and those from outside the Douro valley are striking, and quite noticeable.

There are differences in aging, for instance, even in Portugal. Some Port is aged in the bottle in "reductive aging"; Port aged this way tends to retain the dark red color of the original wine, and becomes mellower in taste. Some Port is aged in wooden barrels. The natural permeability of the wood allows some air to enter the barrels, so that the wine undergoes varying degrees of oxidation; it becomes lighter in color, and since some fluid is lost to evaporation, the liquidity and flavor is intensified. This Port is typically labeled as "Tawny Port," because of the color difference, or sometimes, the label will say "aged in wood." The wood also affects the flavor, lending the Port a "nutty" quality. Tawny Ports tend to be sweet or medium dry, and are usually dessert wines. The usual aging period is seven years, but some Port is aged for 10 or more years. Genuine Port ages exceedingly well; even over 100 years.

Bottle-aged Port produced in the U.S. is typically blended to produce a specific character selected by the producer. It is most often identified as Ruby Port. After the Port has fermented it is stored in tanks to stop further oxidation, and preserve its rich color. It is blended, then bottled, and it doesn't improve with age. This is Port to be purchased and consumed near-term.

Portuguese Vintage Port is an exceedingly difficult and complex subject; there are a number of varieties, and sub-varieties. Essentially, vintage Port means that the Port has been aged for a number of years (ten or more) in the bottle. Such Port is usually not filtered, and is somewhat delicate in terms of storage; it must not be jostled about, for instance, because of the natural sediment. Vintage Ports are usually clearly labeled as Vintage Port, and there's usually a specific bottle date. There is a complicated nomenclature for Vintage Port; Quinta usually means that the wine is from a single vineyard, rather than blended from a number of different grape sources. Producers only make the Port in a year that they feel is of a suitable quality. There's a good introduction to Vintage Port here.