Port from Portugal comes in several styles, which can be divided into two broad categories:
Wines that have matured in sealed glass bottles, with no exposure to air, and experience what is known as “reductive” aging. This process leads to the wine losing its colour very slowly and produces a wine which is smoother on the palate and less tannic.
Wines that have matured in wooden barrels, whose permeability allows a small amount of exposure to oxygen, and experience what is known as “oxidative” aging. They too lose colour, but at a faster pace. If red grapes are used, in time the red colour lightens to a tawny colour – these are known as Tawny (or sometimes Wood) ports. They also lose volume to evaporation (angel’s share), leaving behind a wine that is slightly more viscous and intense.
The IVDP (Instituto dos Vinhos do Douro e Porto) further divides Port into two categories: normal Ports (standard Rubies, Tawnies and White Ports) and Categorias Especiais, Special Categories, which includes everything else.