A free trade zone in Europe is an area within the EU separate from the rest of the territory in which all types of goods can be introduced irrespective of the quantity, nature, origin, source or place of arrival in as much as they respect the prohibitions and restrictions regarding public order, morality, public security, health etc. These goods can remain within the free trade zone until the interested parties decide what to do with the goods. During the course of time they remain within the zone, these goods are not subject to import duties or any other internal taxes or indeed any other of the applicable commercial measures. In this sense, they are essentially areas that for all intents and purposes are not considered to be part of the European Union
Generally speaking the largest of these areas tend to be in port areas where any number of companies set up in order to carry out their business availing at the same time of exemption benefits amongst others. They are therefore very often used by import-export companies but also companies in the assembly sector.
There are currently 4 free trade zones in Spain being Barcelona, Gran Canary, Vigo and Cadiz. If you are a company trading internationally and are thinking of setting up in one of these areas, it is worthwhile to consider the respective benefits of each in terms of such things as additional services etc.
It is also worth bearing in mind that the Canary Islands which is currently in a transition period also has a special tax regime which may provide other advantages for companies that trade internationally.