farm building vs barn

farm building and barn both are nouns.

Word	Noun	Adjective	Verb	Adverb
farm building	Yes	No	No	No
barn	Yes	No	No	No

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

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

Other hyponyms of farm building include chicken coop, coop, hencoop, henhouse, horse barn, stable, stalls.

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