Cask ale

Real ale is the name coined by the Campaign for Real Ale for ale 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 distinction between real and other ales is that real ale is unfiltered and unpasteurized, leaving its yeast alive and slowly fermentating in bottle or keg. This secondary fermentation continues until the ale is served, allowing it to retain its natural carbonation. In contrast, natural carbonation is removed from standard beer and ale during filtering, requiring them to be artificially re-carbonated (and often very 'gassy').

Source: Wikipedia