Cask ale or cask-conditioned beer is the term for unfiltered and unpasteurised beer which is conditioned (including secondary fermentation) and served from a cask without additional nitrogen or carbon dioxide pressure.
What this means is that cask ale doesn’t have much carbonation, and is very creamy. It’s the Platonic Form of beer. Most big cities now have a few pubs serving cask ale, and you should always order whatever is available in a cask. It will almost always be better than anything else.
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Local interest: I recently had the opportunity to visit Toronto’s C’est What, which has no less than five casks at any given time. Highly recommended. Also, check out CASK!, a blog devoted to cask ale in Toronto. They have a list of establishments serving the stuff.