Cask ales seem popular outside of the US (especially in the UK). What distinguishes that style of beer from beer out of a normal keg?
Cask ales are not stored under pressure , and require a pump to transfer the beer from the cask. As the ale is not under pressure, it is also not as heavily carbonated as other beers.
In contrast kegged beer is stored under pressure, and is forced to the tap by pumping gas into the keg (usually carbon dioxide or nitrogen).
Cask beer is not kept under pressure whereas Keg beer is (with CO2 or a nitrogen mixture).
Beers served in Cask vs Keg (w/ CO2) vs Keg (w/ Nitro) will have slightly different tastes and appearances. Cask beer will be a "flatter" due to the lack of pressure keeping the CO2 in solution in the beer. Cask beer can also take on a butterscotch flavor due to the exposure to air. Hoppy beers will be mellowed in the Cask vs in a Keg. Beer in a cask needs to be drank within a few days of opening the cask due to the exposure to oxygen.
Kegs served with CO2 are typically how beers are served. If the keg is using a Nitro tap, the beer will get a smoother milky texture due to the gas and the head will cascade in waves. The beer will last much longer in a keg because of the gas keeping oxygen away from the beer.
Depends on who you ask. Both of these answers are correct but some would take the definition of cask ale a little further by defining it as an ale that is unfiltered and unpasteurised and conditioned (including secondary fermentation) and served from a cask without additional nitrogen or carbon dioxide pressure.
One of our local beer shops has a few cask ales on tap, ran by someone from England. He often scolds beer sold as cask ales which were only placed into the cask after fermentation is completed.