dancer vs folk dancer

dancer and folk dancer both are nouns.

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

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

dancer: a person who participates in a social gathering arranged for dancing (as a ball)
folk dancer: someone who does folk dances

Other hyponyms of dancer include clog dancer, dancing partner, raver, waltzer.

dancer (noun)	folk dancer (noun)
a performer who dances professionally	someone who does folk dances
a person who participates in a social gathering arranged for dancing (as a ball)