Port is both a fortified wine made from the Douro Valley in Portugal or a wine made anywhere in the world, modeled after that example.
True Port is made from several red varieties and is fortified with brandy or distilled grape sprits, which cause the fermentation to stop and also increases alcohol. Port checks in at 17% - 20% alcohol in most cases. Because of this alcohol, ports can age well- some as long as four or five decades. Most younger ports are quite sweet, while those that have been aged in wood for several years are drier.
Several other countries also produce a wine they label as port. The best examples are from Australia and the United States.