Wild Ales are complex sour beers fermented with so-called "wild" yeasts, lactic acid bacteria and various bacteria. They are similar in taste to Belgian lambic. Wild Ales are often barrel-aged, and some are aged on fruit or herbs.
Central to the flavor of Wild Ales are the different strands of Brettanomyces yeasts. They produce a unique flavor that is described as funky and complex. Fermentation with different yeasts and bacteria can make these beers very sour. However, by aging in barrels for long periods of time or blending different brews, Wild Ales develop a balanced, complex and usually mildly sour taste.
This beer style in its current form originated in the USA. There are many breweries there that brew Lambic-inspired beers and age them on local or regional fruit. However, the history of wild ales goes back much further. After all, in earlier eras, spontaneously fermented beer was consistently brewed without the extra addition of yeast. And Brettanomyces yeasts were part of those wild yeasts that provided fermentation.
Many breweries also use local wild yeasts to brew wild ales. Take Mikkeller Baghaven, for example, which describes its beers as Danish Wild Ales themselves. The beers brewed there are often made with exclusively Danish ingredients, and that includes the yeast cultures.