folk song vs fado

folk song and fado both are nouns.

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

As nouns, fado is a hyponym of folk song; that is, fado is a word with a more specific, narrower 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
fado: a sad Portuguese folksong

Other hyponyms of folk song include blues.

folk song (noun)	fado (noun)
a song that is traditionally sung by the common people of a region and forms part of their culture	a sad Portuguese folksong