Unfiltered beer (usually ale), that is racked (transferred) into casks, krausened (carbonated), sealed and then undergoes a slight final fermentation in the cask. Often finings (clarifying agents) will be added to help the beer drop bright—when yeast cells naturally flocculate (clump) and settle at the bottom of a vessel.
The net result is a beer that has a much gentler level of carbonation, a rounder mouthfeel, and since it is unfiltered, usually shows a slightly more complex flavor and aromatic profile. It is often slightly cloudy, and is best served at cellar temperature (around 55°F). It’s considered “live” beer since it is unfiltered and has live yeast in the cask (which is how we get the final fermentation and carbonation).