poster vs flash card

poster and flash card both are nouns.

Word	Noun	Adjective	Verb	Adverb
poster	Yes	No	No	No
flash card	Yes	No	No	No

As nouns, flash card is a hyponym of poster; that is, flash card is a word with a more specific, narrower meaning than poster:

poster: a sign posted in a public place as an advertisement
flash card: a card with words or numbers or pictures that is flashed to a class by the teacher

Other hyponyms of poster include show bill, show card, theatrical poster, flashcard.

poster (noun)	flash card (noun)
a horse kept at an inn or post house for use by mail carriers or for rent to travelers	a card with words or numbers or pictures that is flashed to a class by the teacher
a sign posted in a public place as an advertisement
someone who pastes up bills or placards on walls or billboards