girl vs farm girl

girl and farm girl both are nouns.

Word	Noun	Adjective	Verb	Adverb
girl	Yes	No	No	No
farm girl	Yes	No	No	No

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

Other hyponyms of girl include Campfire Girl, flower girl, moppet, schoolgirl, Scout, flower boy.

girl (noun)	farm girl (noun)
a young female	a girl who has grown up on a farm
a girl or young woman with whom a man is romantically involved
a youthful female person
a friendly informal reference to a grown woman
a female human offspring