What is the difference between cask ale and keg beer?
Whether the beer finishes fermenting in the container it is served from, and whether it is dispensed using added gas — and those two differences produce distinctly different drinks.
Cask ale (real ale). Beer that undergoes secondary fermentation in the cask from which it is served. It is unfiltered and unpasteurised, and live yeast remains in it.
Carbonation is natural, produced by that ongoing fermentation, which is why cask ale is comparatively lightly carbonated.
Served without added gas. Drawn by a handpump or by gravity, with air replacing the beer as it is drawn.
Served at cellar temperature — around 11–13°C, which is cool rather than warm. The "warm British beer" characterisation is mostly a comparison with heavily chilled lager.
It is a living product, which is the crucial practical consequence. Once the cask is broached and air enters, it begins to deteriorate and is typically at its best for only a few days. Cask ale depends heavily on cellar skill — conditioning, temperature, venting and turnover — which is why the same beer can be excellent in one pub and poor in another.
Keg beer. Filtered and usually pasteurised, so it is stable — fermentation is complete and the product does not change in the container.
Carbonated with added gas, and dispensed under pressure using CO₂ or a nitrogen mix. Nitrogen produces the fine, dense, creamy head familiar from stouts, because nitrogen bubbles are smaller and less soluble.
Served colder and with higher carbonation.
Consistent and long-lived, which is why it dominates commercially — a keg does not require judgement to serve well.
"Craft keg" describes modern keg beer that is frequently unpasteurised and sometimes unfiltered, which blurs the traditional distinction considerably.
Bottle-conditioned beer applies the cask principle in a bottle, with live yeast and sediment.
Which is better is a matter of style rather than quality. Cask suits lower-carbonation, subtler beers where warmth releases aroma; keg suits heavily hopped, high-carbonation and cold-served styles, where cask conditions would flatten the intended character.