fun vs pun

fun and pun both are nouns.

Word	Noun	Adjective	Verb	Adverb
fun	Yes	No	No	No
pun	Yes	No	Yes	No

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

fun: verbal wit or mockery (often at another's expense but not to be taken seriously)
pun: a humorous play on words

fun (noun)	pun (noun)
activities that are enjoyable or amusing	a humorous play on words
violent and excited activity
a disposition to find (or make) causes for amusement
verbal wit or mockery (often at another's expense but not to be taken seriously)

fun (verb)	pun (verb)
	make a play on words