folk song vs folk music

folk song and folk music both are nouns.

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

As nouns, folk music is a hypernym of folk song; that is, folk music 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
folk music: the traditional and typically anonymous music that is an expression of the life of people in a community

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

folk song (noun)	folk music (noun)
a song that is traditionally sung by the common people of a region and forms part of their culture	the traditional and typically anonymous music that is an expression of the life of people in a community