Port is a fortified wine, meaning that additional alcohol has been added to what might otherwise be a normal wine.
While wines were being produced in Portugal since the craft was first brought through by the Romans, the legecy of Port is often traced to the mid 17th century. The British found themselves cut off from their supply of French wines due to unforunate wars, and so turned to other trading partners in an effort to maintain their cellars. The wines of Portugal were not as refined as those of France, and the long sea voyage did nothing to improve their character. It was found that by adding a goodly amount of Brandy to the casks of wine prior to shipment that they would arrive at their destination in much better shape. Eventually, this practice led to adding the brandy during the fermentation process instead of afterwards. This resulted in stopping the fermentation while the wine was still sweet, fruity, and strong in flavor. Various socio-political events followed which eventually resulted in the very strictly regulated product that we have today.
The ultimate product from any of the Port Houses of Portugal is a Vintage Port. Unlike a vintage wine, vintage ports don't happen every year. Each port producer must make the decision as to if a particular year is producing a product of high enough quality to "declare a vintage", and this can only be done only after the wine has spent two years aging. Once that is done, the producer must follow very carefully controlled guidelines in order maintain the vintage designation.
All Vintage Ports are bottled between July 1st of the second year from origiinal harvest and June 30th of the third year from original harvest. There are a wide number of additional regulations that need to be met in order for a port to be a "Vintage" port, including the submission of two "reference" bottles to the Instituto do Vinho do Porto (IVP) which at any point in the future can be opened and compared against any other bottle from that same vintage, and if serious discrepencies are detected, that particular vintage designation can be revoked.
A Vintage Port will not be ready to drink for at least 10 or more years. Many claim that the port should be at least as old as the drinker.
Late Bottle Vintage (LBV)
Similar to a Vintage Port, but usually not of the same quality. As the name implies, it is bottled after spending a longer time in oak casks, between four to six years. Also like Vintage Port, it needs to be registered with the IVP, and its bottling and labeling are strictly controled.
Like a Vintage Port, an LBV port must contain only grapes harvested from a particular year, but unlike Vintage Port, it is bottled ready to drink.
As the name implies, these ports are of a deep red color, and are characteristically young and fruity in flavor. They are almost always a blending of various years harvests and they are ready for drinking immediately.
Similar to a Ruby Port, but are aged longer in their oak casks. This mellows both their color and flavor.