signboard vs billboard

signboard and billboard both are nouns.

Word	Noun	Adjective	Verb	Adverb
signboard	Yes	No	No	No
billboard	Yes	No	No	No

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

signboard: structure displaying a board on which advertisements can be posted
billboard: large outdoor signboard

Other hyponyms of signboard include hoarding, sandwich board, scoreboard, shingle.

signboard (noun)	billboard (noun)
structure displaying a board on which advertisements can be posted	large outdoor signboard