In short, sour beers are beers that are aged in barrels for as long as three years until they develop the sought after tart and tangy flavor that the beer maker is looking for. They each add a different variety of sour bacteria and yeast which hang out in the barrels and end up soaking up the sugars in the beer leaving intense flavors which can range from sharp and fruity to earthy and spicy. The art is in the combination of different yeasts (with Brettanomyces yeast being a popular one) and bacteria and the time in which they're allowed to hang out and ferment.
Traditional sour beers are more popular in Belgium where you'll see lambics of all varieties. But I think the reason more and more friends have been seeking out bars with sour beers on tap is because there are some great folks brewing it stateside these days, and it's starting to gain some domestic buzz. Drinking a sour beer seems much more of an experience than drinking your go-to lager because the flavor profile isn't immediately expected and more and more chefs are trying to get them on the menu because they enhance the flavor of food in a much different way that your traditional IPA or other hoppy beer.
So have you tried any good domestic sour beers? What are your favorites?
(Images: Ben + Sam via Flickr)