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An important distinction between beer styles is often overlooked in descriptions and critical discussions. At the highest level, all beer is made in one of two styles: lager or ale. The determinant is the type of yeast used in fermentation - 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 can add some fruity, distinctive flavour and aroma characters 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, often 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, for example wheat, provides some classic and superb wheat beers, quite different to the mainstream light gold fizzy liquid that is often associated with the label lager. For more information on the lager brewing process and its origins see the separate guide entry for 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 the 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 - 13°C) 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 - labelling darker brews as ales, stouts (such as Guinness or porters) in the belief, or in an attempt at least to kid drinkers, that colour determines the nature of the drink.