cave vs grotto

cave and grotto both are nouns.

Word	Noun	Adjective	Verb	Adverb
cave	Yes	No	Yes	No
grotto	Yes	No	No	No

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

cave: a geological formation consisting of an underground enclosure with access from the surface of the ground or from the sea
grotto: a small cave (usually with attractive features)

Other hyponyms of cave include cavern, cove, grot.

cave (noun)	grotto (noun)
a geological formation consisting of an underground enclosure with access from the surface of the ground or from the sea	a small cave (usually with attractive features)

cave (verb)	grotto (verb)
explore natural caves
hollow out as if making a cave or opening