Sour beer is beer which has an intentionally acidic, tart or sour taste. The most common sour beer styles are Belgian: lambics, gueuze and Flanders red ale. Although now every country is getting in on the act reviving styles some thought to be lost, Gose and Berlinerwiesse, for example, which both have their own distinctive yet very sour tastes.
At one time, all beers were sour to some degree. As pure yeast cultures were not available, the starter used from one batch to another usually contained some wild yeast and bacteria. Unlike modern brewing, which is done in a sterile environment to guard against the intrusion of wild yeast, sour beers are made by intentionally allowing wild yeast strains or bacteria into the brew.
The most common yeasts/bacteria used to intentionally sour beer are Lactobacillus, Brettanomyces, and Pediococcus. Because of the uncertainty involved in using wild yeast, the sour beer brewing process is extremely unpredictable, but incredible exciting. The beer takes months to ferment and can take years to mature. It's truely the epitome of artisanal brewing.