cask vs beer keg

cask and beer keg both are nouns.

Word	Noun	Adjective	Verb	Adverb
cask	Yes	No	No	No
beer keg	Yes	No	No	No

As nouns, beer keg is a hyponym of cask; that is, beer keg is a word with a more specific, narrower meaning than cask:

Other hyponyms of cask include beer barrel, butt, hogshead, keg, pickle barrel, shook, tun, wine barrel, wine cask.

cask (noun)	beer keg (noun)
a cylindrical container that holds liquids	a barrel that holds beer
the quantity a cask will hold