placard vs poster

placard and poster both are nouns.

Word	Noun	Adjective	Verb	Adverb
placard	Yes	No	Yes	No
poster	Yes	No	No	No

As nouns, placard and poster are synonyms defined as:

Other synonyms of placard include bill, card, notice, posting.

placard (noun)	poster (noun)
a sign posted in a public place as an advertisement	a horse kept at an inn or post house for use by mail carriers or for rent to travelers
	a sign posted in a public place as an advertisement
	someone who pastes up bills or placards on walls or billboards

placard (verb)	poster (verb)
publicize or announce by placards
post in a public place