What exactly is a Firkin?

A firkin, derived from the Dutch word vierdekijn (meaning fourth), is actually a small barrel of beer that is one-fourth the size of a stand full barrel of beer; as opposed to a standard beer barrel, which contains 30.96 gallons, a firkin full of cask ale will contain a volume of 10.79 gallons. A firkin is typically dedicated to housing “true ale” (i.e., cask-conditioned ale), or beer that has not been cold-filtered, pasteurized and carbonated by outside equipment. The ale beer that is housed inside the firkin is naturally carbonated by its resident yeast and its ingredients have not been processed in any way outside of simple fermentation by the yeast

Here is a link to a short You Tube video showing what a firkin is: