folk vs riffraff

folk and riffraff both are nouns.

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

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

folk (noun)	riffraff (noun)
the traditional and typically anonymous music that is an expression of the life of people in a community	disparaging terms for the common people
people in general (often used in the plural)
a social division of (usually preliterate) people
people descended from a common ancestor