promissory note vs IOU

promissory note and IOU both are nouns.

Word	Noun	Adjective	Verb	Adverb
promissory note	Yes	No	No	No
IOU	Yes	No	No	No

As nouns, IOU is a hyponym of promissory note; that is, IOU is a word with a more specific, narrower meaning than promissory note:

promissory note: a promise to pay a specified amount on demand or at a certain time
IOU: an informal debt instrument; representing `I owe you'

Other hyponyms of promissory note include demand note, note receivable, note payable, municipal note, time note.

promissory note (noun)	IOU (noun)
a promise to pay a specified amount on demand or at a certain time	an informal debt instrument; representing `I owe you'