candidate vs write-in

candidate and write-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
candidate	Yes	No	No	No
write-in	Yes	No	No	No

As nouns, write-in is a hyponym of candidate; that is, write-in is a word with a more specific, narrower meaning than candidate:

candidate: a politician who is running for public office
write-in: a candidate for public office whose name does not appear on the ballot and so must be written on the ballot by the voters

Other hyponyms of candidate include dark horse, favorite son, running mate, spoiler, stalking-horse, write-in candidate.

candidate (noun)	write-in (noun)
someone who is considered for something (for an office or prize or honor etc.)	a vote cast by writing in the name of a candidate who is not listed on the ballot
a politician who is running for public office	a candidate for public office whose name does not appear on the ballot and so must be written on the ballot by the voters