What's the difference between a cask beer and a kegged beer? 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).