title_body: What is the German Beer Purity Law?
upvoted_answer: The German Beer Purity Law, also know as the Reinheitsgebot, dictates what ingredients may be used to create beer in Germany: barley, hops, and water.  It dates back to 1487, which is why you may notice the omission of yeast: it hadn't been recognized as an ingredient yet.  The law was removed from the books in 1993, and replaced by another similar law which allowed yeast, sugar, and some of the more common brewing ingredients.
  
  In the case of a US brewery making such a claim, it's a sort of advertisement toward the "purity" of their product, i.e., they don't use adjuncts (rice, etc) or other flavorings (fruit, spices) in their beer.