In the 18th century, Portuguese wines, when transported to London regularly, turned into vinegar. To avoid this problem a British merchant had the idea of adding alcohol to the wines. This is how Port wine as we know it today was born. 

Port wine was so successful that to preserve the quality the Portuguese government had to introduce regulations. In 1756 the first Protected Designation of Origin was created. 

England, is not a wine producer and was reluctant to buy its wine in France and Spain, countries that were often enemies. Thus, England very quickly found in Portugal the ideal partner for sourcing wine and especially Port wine . 

Ruby Port used cask & barrels:

Ruby Port wines are put into oak barrels to improve their quality before bottling

Ruby barrels bring sweetness and strong red fruit aromas.

Tawny Port used cask & barrels:

The Tawny must be aged in oak barrels for at least 5 years. Some can be aged up to 40 years or more. 

The contributions of these second hand casks & barrels are: oxidative notes, chocolate, and candied fruit aromas.