roar vs holler

roar and holler both are nouns.

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

As nouns, roar and holler are synonyms defined as:

Other synonyms of roar include bellow, bellowing, holla, hollering, hollo, holloa, roaring, yowl.

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

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

roar (noun)	holler (noun)
a very loud utterance (like the sound of an animal)	a very loud utterance (like the sound of an animal)
a deep prolonged loud noise	a small valley between mountains
the sound made by a lion

roar (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
utter words loudly and forcefully
make a loud noise, as of animal
act or proceed in a riotous, turbulent, or disorderly way