bulletin vs write up

bulletin and write up both are nouns.

Word	Noun	Adjective	Verb	Adverb
bulletin	Yes	No	Yes	No
write up	Yes	No	Yes	No

As nouns, write up is a hypernym of bulletin; that is, write up is a word with a broader meaning than bulletin:

bulletin: a brief report (especially an official statement issued for immediate publication or broadcast)
write up: a short account of the news

Other hypernyms of bulletin include account, news report, report, story.

bulletin (noun)	write up (noun)
a brief report (especially an official statement issued for immediate publication or broadcast)	a short account of the news

bulletin (verb)	write up (verb)
make public by bulletin	bring to public notice by writing, with praise or condemnation
	put into writing; write in complete form