Cask-conditioned ale, also known as cask ale, is ale that is finished in a cask, or firkin, unfiltered and unpressurized. Draft beer is filtered for longer shelf life and ease of plumbing, then it is pressurized with CO2 to carbonate it and to push it to the tap. Cask ale is rare for many reasons, including its shorter shelf-life and the fact that it cannot be drawn through a regular tap.
So...maybe cask-conditioned ale is like the buggy-whip, superceded by better technology and no longer of value? Not so!
Cask-conditioned ale has a noticeably fresher, livelier taste than draft or bottle beer. It requires more care and skill to serve cask ale, but the beerhound cognoscenti know about it and seek it out.
Cask ale tastes livelier because it's still alive! The yeast remains in it, providing all the carbonation it will ever have. One reason the firkin must be handled so gingerly is to keep from stirring up the yeast, which takes a long time to settle.
I served a firkin of Mayflower Ale at an Old Colony Club Past Presidents Night Gala. It was a great hit, but it was not without some difficulties: I had to learn a lot about how to handle it and how to talk about it - when to tap it with the soft spile and the hard spile, how to knock the bung and add the tap.
For a one-night event, you might see the firkin on a counter with a basic tap in place. At a pub that regularly serves cask-conditioned ales, you'll see a beer engine that the server pulls on once or twice to draw the ale from the firkin to the glass.
You can learn more about cask ale, including where to find it, from NERAX, the New England Real Ale Exhibition, hosted by CASC, the Cask-conditioned Ale Support Campaign. There is also information at the graphically-challenged Alex Hall's USA Beer Pages, but it is sometimes out of date.
If you know of a reliable place to get cask-conditioned ale, please mention it in the comments section below.