Protected Designation of Origin (PDO) is defined as the name of a region or a specific place used to describe a wine product which denotes that the grapes have come exclusively from that area and are only of the Vitis vinifera genus, and the production of the wine takes place in the named area.
Protected Geographical Indication (PGI) is slightly less restrictive and is defined as the name of a region, a specific place or a country used to describe a wine product which possesses a specific quality, reputation or other characteristics attributable to the geographical origin, that at least 85% of grapes used for its production have come from that area and are of the Vitis Vinifera genus or a cross of Vitis Vinifera and another genus of Vitis and the production of the wine takes place in the named area.
These are general European terminology, but each country will have their own words to say more or less the same thing, which basically tries to protect the region and the producers in that region.
In Spain, the categories are Vino de Mesa (table wine), Vino de la Tierra, VT or VdlT(“wine of the country”), Vinos de Calidad con Indicación Geográfica (VCIG or VC), Denominación de Origen, or DO, Denominación de Origen Calificada, or DOC (sometimes referred to as DOCa, or DOQ in Catalunya), Vino de Pago, or VP
Vino de Mesa is the lowest rung on Spain’s wine quality ladder.
VT is like the Vins de Pays for French wines. It doesn’t necessarily mean the wine has no quality, but just that the wine didn’t follow the rules and restrictions (and sometimes the quality level) which a higher qualification rules.
VCIG is the European PGI. It’s like the French Vin Délimité De Qualité Supérieure (VDQS), or the Italians IGT, which is basically a holding place for aspiring a higher qualification.
DO and DOC (DOCa, DOQ) are the highest appelations in Spain, comparable to France’s AOC (Appellation d’Origine Contrôlée). All DOs have regulatory bodies responsible for creating the definition of each DO.
VP is a new concept with an entirely different method of classifying quality. Pago means vineyard, so the simple explanation of what constitutes a DO Pago is that it is a single estate wine.