poster girl vs woman

poster girl and woman both are nouns.

Word	Noun	Adjective	Verb	Adverb
poster girl	Yes	No	No	No
woman	Yes	No	No	No

As nouns, woman is a hypernym of poster girl; that is, woman is a word with a broader meaning than poster girl:

Other hypernyms of poster girl include poster child, adult female.

poster girl (noun)	woman (noun)
a female poster child	women as a class
	an adult female person (as opposed to a man)
	a female person who plays a significant role (wife or mistress or girlfriend) in the life of a particular man