barn vs cow barn

barn and cow barn both are nouns.

Word	Noun	Adjective	Verb	Adverb
barn	Yes	No	No	No
cow barn	Yes	No	No	No

As nouns, cow barn is a hyponym of barn; that is, cow barn is a word with a more specific, narrower meaning than barn:

barn: an outlying farm building for storing grain or animal feed and housing farm animals
cow barn: a barn for cows

Other hyponyms of barn include byre, cowbarn, cowhouse, cowshed, tithe barn.

barn (noun)	cow barn (noun)
an outlying farm building for storing grain or animal feed and housing farm animals	a barn for cows
(physics) a unit of nuclear cross section; the effective circular area that one particle presents to another as a target for an encounter