briefs vs underpants

briefs and underpants both are nouns.

Word	Noun	Adjective	Verb	Adverb
briefs	Yes	No	No	No
underpants	Yes	No	No	No

As nouns, underpants is a hypernym of briefs; that is, underpants is a word with a broader meaning than briefs:

briefs: (used in the plural) short tight-fitting underpants (trade name Jockey shorts)
underpants: an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin

briefs (noun)	underpants (noun)
(used in the plural) short tight-fitting underpants (trade name Jockey shorts)	an undergarment that covers the body from the waist no further than to the thighs; usually worn next to the skin