Cask ale

Real ale is the name coined by the Campaign for Real Ale for beer that is "brewed from traditional ingredients, matured by secondary fermentation in the container from which it is dispensed, and served without the use of extraneous carbon dioxide". Cask and bottle-conditioned beers are referred to as real ale by CAMRA, as both fit its description of beers served from a container in which they have undergone secondary fermentation. The fundamental distinctions between real and other ales is that real ale is required to be unfiltered, unpasteurized and served either from a beer engine without the use of extraneous carbon dioxide, or via bottle conditioning. Real ale is not filtered and the yeast is still present and living in the container from which the real ale is served, although it will have settled to the bottom and is usually not poured into the glass. The natural carbon dioxide is lost during filtration so filtered beer has to be artificially re-carbonated. This can make the beer very 'gassy'. Because the yeast is still present and alive in real ale, a slow process of secondary fermentation continues in the cask or bottle on the way to the consumer, allowing the beer to retain its natural carbonation. Another distinction is that real ale should be served without the aid of added carbon dioxide, or "top pressure" as it is commonly known. Common dispensing methods are the handpump, or "by gravity" direct from the cask. Electric pumps are occasionally seen, especially in the Midlands and Scotland. Water pumps, powered by mains water pressure, were the traditional means of dispensing draught beer in Scotland but this method is discontinued.

Source: Wikipedia