Spanish people are famous to drink very often, but socially mainly, over a big and long meal or many tapas, thus it is very seldom to see somebody drunk. Wine and beer are the most typical drinks. In Madrid in many tapas places they follow the Andalucian tradition that customers buy the drinks and the owner will always serve something to eat as a starter as an invitation evry time they order a drink. Still it is better to ask if it is the case in advance, to avoid misunderstandings.
The beer served at the moment from a barrel is called "caña" and is usually served cold. They mix it often with sparkling water or with lemon sprite. If you want to try them, you should ask for "una clara/caña con gasosa" and "caña con limón".
A typical drink is the summer red wine mix "tinto de verano". To the red wine is added orange juice, cinnamon, sugar, lemon, mint, red wine and ice cubes. At some more traditional places, people still meet for a vermouth, cider or txacolí (young sparkling white wine from the Basque region). Make sure to try a good Albariño (white fruity wine), as well as good Rioja and Ribera del Duero wines. If you want to surprise somebody, share with them a glass of blue Spanish wine Marqués de Alcántara or Gik. Its turquoise colour is completely natural and comes from the natural antioxidants obtained by a special mixture of blue and white grapes.
The local alternative to Champagne is called Cava and often can compete with the best Champagnes we know.
You may find an interesting suggestion for the youngest or those who avoid alcohol, the summer drink "Horchata" served cold with sugar, it is known to be served best at the Alboraya Cafeterias with ice cream.