kaiser roll vs bun

kaiser roll and bun both are nouns.

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

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

kaiser roll: rounded raised poppy-seed roll made of a square piece of dough by folding the corners in to the center
bun: small rounded bread either plain or sweet

Other hypernyms of kaiser roll include roll.

kaiser roll (noun)	bun (noun)
rounded raised poppy-seed roll made of a square piece of dough by folding the corners in to the center	small rounded bread either plain or sweet