barn vs farm building

barn and farm building both are nouns.

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

As nouns, farm building is a hypernym of barn; that is, farm building is a word with a broader meaning than barn:

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

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