sweet roll vs bun

sweet roll and bun both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweet roll	Yes	No	No	No
bun	Yes	No	No	No

As nouns, bun is a hypernym of sweet roll; that is, bun is a word with a broader meaning than sweet roll:

sweet roll: any of numerous yeast-raised sweet rolls with our without raisins or nuts or spices or a glaze
bun: small rounded bread either plain or sweet

Other hypernyms of sweet roll include roll.

sweet roll (noun)	bun (noun)
any of numerous yeast-raised sweet rolls with our without raisins or nuts or spices or a glaze	small rounded bread either plain or sweet