I get asked a lot what the heck a firkin is and why do people serve beer in one. Well, here is the answer.
A firkin is a unit of measure and also the size of a particular kind of keg used for cask conditioning; one quarter of a barrel, 72 pints, 9 imperial gallons. Firkins can be wooden or metal. The most curious thing about them is the manner in which they are tapped. As you can see, in order to keep air out, the keg is laid on its side and the spout is driven in horizontally. It can get a bit messy if not done correctly.
A breathing tube is inserted in the bung which is now on top to enhance the flow of beer using only gravity. CO2 is not used to carbonate or push the beer out.
Thus you get a naturally fermented and carbonated beer in a small batch. Many think beer tastes much better this way. In fact, there are entire beer festivals dedicated to only Cask Conditioned beers.
And now you know.