howl vs holler

howl and holler both are nouns.

Word	Noun	Adjective	Verb	Adverb
howl	Yes	No	Yes	No
holler	Yes	No	Yes	No

As verbs, holler is a hypernym of howl; that is, holler is a word with a broader meaning than howl:

Other hypernyms of howl include call, cry, hollo, scream, shout, shout out, squall, yell.

howl (noun)	holler (noun)
a long loud emotional utterance	a very loud utterance (like the sound of an animal)
a loud sustained noise resembling the cry of a hound	a small valley between mountains
the long plaintive cry of a hound or a wolf

howl (verb)	holler (verb)
laugh unrestrainedly and heartily	complain
make a loud noise, as of wind, water, or vehicles	utter a sudden loud cry
emit long loud cries	shout out
cry loudly, as of animals