stranger vs foreigner

stranger and foreigner both are nouns.

Word	Noun	Adjective	Verb	Adverb
stranger	Yes	No	No	No
foreigner	Yes	No	No	No

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

stranger: anyone who does not belong in the environment in which they are found
foreigner: someone who is excluded from or is not a member of a group

Other hyponyms of stranger include outsider.

stranger (noun)	foreigner (noun)
anyone who does not belong in the environment in which they are found	a person who comes from a foreign country; someone who does not owe allegiance to your country
an individual that one is not acquainted with	someone who is excluded from or is not a member of a group