Beer, ale and lager

Nowadays the largest division in beer is based on the species of yeast used in its production. "Beer" is the general term used for fermented alcoholic beverages made from malted barley and flavoured with hops, which is divided into "lagers" made with the yeast Saccharomyces pastorianus and "ales" made with Saccharomyces cerevisiae. 

It wasn't always this way. At one point if hops were used was what the definition was based around. "Ales" were made without hops and only when hops were used was the drink called "beer". And if I remember rightly the beer writer beer writer Michael Jackson divided beer into three main types: ales, lagers and wheat beers. Sometimes "stout" is considered a separate category too.

So to varying degrees yeast, hops and grains have been used to classify beer into different kingdoms. None are particularly satisfactory though, and the flaws in the current yeast based system are now becoming more apparent.

Beer styles from around the world are easily available than they've ever been and dividing them simply into ales or lagers often provides little useful information. A kölsch may be made with ale yeast but the beer surely has more in common with German lagers than British ales. And speaking of lagers some are actually made with ale yeast and some beers sold as ales are made with lager yeast.

Classifying beer is a difficult task and inventing an ever greater profusion of beer styles can add to the confusion too. But yeast species is not necessarily the most important thing about a beer, it's just another factor in the ingredients and processes that determine how the beer tastes.