Winemeister

Of the three top wine producing countries (France  and Italy  being the other two), Spain has the lowest production in hl per year, but the highest amount of area under vines (see statistics  page). Its low productivity is mostly caused by the extreme conditions in most of Spain's wine growing regions. Central Spain is notorious for its water problems and only some of the northern regions get sufficient amounts of water to guarantee stable yields.

The good news is that due to the severe conditions the wine is growing under, there are some exceptional red wine vintages expecially from the southern regions. The bad news is because of the fluctuations to the microclimate year over year, it is very hard to predict which ones they are in any given vintage.

Since joining the European Union, Spain has introduced similar quality controls as the other European wine growing countries. The system is administered by the INDO  ("Instituto Nacional de Denominaciones de Origen") government agency. The INDO knows four basic distinctions of wine quality:

Vino de Mesa (VdM) is the equivalent of what is called a "table wine" in most EU countries and captures any wine that does not qualify for the higher quality standards. However, it does not necessarily mean you are buying bad wine - for example, any wine could be disqualified from the other categories by non-traditional blending or other process violations that do not always reflect on the quality of the wine. Therefore many of Spain's expensive wines are sold as VdM wines.
Vino de la Tierra (VdlT) guarantees that the grapes going into the wine are coming from the region that is identified on the bottle and that the wine was made according to local traditions usually captured in the local wine making laws.
Denominacion de Origen (DO) wines are the equivalent of the Italian DOC  or the French AC  system. Each DO has an assigned regulator ("Consejo Regulador") who determines how wine is grown, made and marketed for any given region.
*Denominacion de Origen Califada" (DOCa) is a distinction introduced in the 1990s and it resembles the Italian DOCG  system. Rioja  is the most prominent type of wine in this category.
According to the INDO, there are currently 53 regions that are recognized as official DO regions:

Abona
Alella
Alicante
Almansa
Ampurdan-Costa Brava
Bierzo
Binissalem
Bullas
Calatayud
Campo de Borja
Cariñena
Chacolí de Guetaria
Chacolí de Vizcaya
Cigales
Conca de Barbera
Condado de Huelva
Costers del Segre
El Hierro
Jerez-Xéres-Sherry, Manzanilla Sanlucar de Barrameda Jumilla
La Mancha
La Palma
Lanzarote
Málaga
Manchuela
Mondéjar
Méntrida
Monterrei
Montilla-Moriles
Navarra
Penedés
Pla de Bages
Pla i Llevant
Priorato
Rias Baixas
Ribeira Sacra
Ribeiro
Ribera del Duero
Ribera del Guadiana
Rioja
Rueda
Somontano
Tacoronte-Acentejo
Tarragona
Terra Alta
Toro
Utiel-Requena
Valdeorras
Valdepeñas
Valencia
Valle de la Orotava
Vinos de Madrid -
Ycoden-Daute-Isora
Yecla