h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

An important distinction between beer styles is often overlooked in descriptions and crtitical discsussions.

At the highest level, all beer is made in one of 2 styles: lager or ale. The determinant is the type of yeast used in fermenrtation - not the colour, weight, sweetness or flavour.

Ales were the earliest forms of beer. They are fermented at warm temperatures by yeasts that float on the surface of the wort (unfermented fluid). These top fermenting yeasts have a fruity, distinctive flavour of their own, and the flavours of the other ingredients integrate with these flavours at the higher fermentation temperatures. This is why ales should (and most do) have more complex, fruity, flavours and aromas than most lagers.

Lagers are fermented at much lower temperatures, and the yeast falls to the bottom of the fermenting vessel. Bottom fermenting yeasts add little of their own flavour to the beer (the cooler temperatures may also restrain those fruity characters). This is why most lagers are clean flavoured and refreshing - but not necessarily simple or light. Use of complex malts, hops, other flavourings and fermentable material (eg wheat) provide some classic and superb beers - quite different to the mainstream light gold fizzy liquid that is often associated with the label lager.

In the context of "real ale" - only ale can be real as the key thing about real ale is that the beer undergoes secondary fermentation (ie gets its sparkle) in bottle or cask - in the same way as champagne in the wine world. This doesn't happen at the low temperatures preferred by lager yeasts.

The fermentation temperature is also traditionally reflected in the serving temperature of the beers. Thus English real ales are served at cellar temperature (around 10 - 13C) rather than chilled. Lagers show their characters at the cooler temperature but the fruity aromatics and yeastiness of great ale is killed by chilling.

Stouts and porters are traditionally ales. Many "new world" breweries use the more controllable lager yeasts on all their beers - labeling darker brews as ales, stouts or porters - in the belief, or in an attempt to kid drinkers, that the reference ale, stout, or porter has something to do with colour.

well, i guess in belgium (of course), beer styles do not just consist of lagers and ales.
most of the beers over here are classifiable in lagers or ales, of course. but what to do with beers of mixed fermentation (both bottom and top fermentation, like rodenbach), mixed beers (as sold in 1 bottle, like young beer with matured beer, beer with juniper), and beers of spontanious fermentation (like gueuze and kriek)?
well, if you happen to be around, just have a taste of these ones.

“What About Stout?” is an article that compares Dublin’s Guinness to Murphy’s and Beamish, its Corkonian rivals. It attempts to define beer styles and discusses weird drinks like Oyster stouts made from seafood. Guinness is good for you? There’s a comment board where you can agree or disagree about the conclusions.
http://blastmagazine.com/the-magazine/culturefashion/food-and-drink/2010/02/what-about-stout/