folk song vs song

folk song and song both are nouns.

Word	Noun	Adjective	Verb	Adverb
folk song	Yes	No	No	No
song	Yes	No	No	No

As nouns, song is a hypernym of folk song; that is, song is a word with a broader meaning than folk song:

folk song: a song that is traditionally sung by the common people of a region and forms part of their culture
song: a short musical composition with words

Other hypernyms of folk song include vocal, ethnic music, folk, folk music.

folk song (noun)	song (noun)
a song that is traditionally sung by the common people of a region and forms part of their culture	the act of singing
	a short musical composition with words
	the characteristic sound produced by a bird
	a distinctive or characteristic sound
	a very small sum