Keg beer is often filtered and/or pasteurised, both of which are processes that render the yeast inactive.

In brewing parlance, a keg is different from a cask. A cask has a tap hole near the edge of the top, and a spile hole on the side used for conditioning the unfiltered and unpasteurised beer. A keg has a single opening in the centre of the top to which a flow pipe is attached. Kegs are artificially pressurised after fermentation with carbon dioxide or a mixture of carbon dioxide and nitrogen gas.

Keg has become a term of contempt used by some, particularly in Britain, since the 1960s when pasteurised draught beers started replacing traditional cask beers.

Keg beer was replacing traditional cask ale in all parts of the UK, primarily because it requires less care to handle. Since 1971, CAMRA has conducted a consumer campaign on behalf of those who prefer traditional cask beer. CAMRA has lobbied the British Parliament to ensure support for cask ale and microbreweries have sprung up to serve those consumers who prefer traditional cask beer.

Pressurised CO₂ in the keg's headspace maintains carbonation in the beer. The CO₂ pressure varies depending on the amount of CO₂ already in the beer and the keg storage temperature. Occasionally the CO₂ gas is blended with nitrogen gas. CO₂ / nitrogen blends are used to allow a higher operating pressure in complex dispensing systems.