signboard vs hoarding

signboard and hoarding both are nouns.

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

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

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

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

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