fur vs underfur

fur and underfur both are nouns.

Word	Noun	Adjective	Verb	Adverb
fur	Yes	No	No	No
underfur	Yes	No	No	No

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

fur: dense coat of fine silky hairs on mammals (e.g., cat or seal or weasel)
underfur: thick soft fur lying beneath the longer and coarser guard hair

Other hyponyms of fur include undercoat.

fur (noun)	underfur (noun)
dense coat of fine silky hairs on mammals (e.g., cat or seal or weasel)	thick soft fur lying beneath the longer and coarser guard hair
a garment made of animal pelts or synthetic fur
the dressed hairy coat of a mammal