Poetry can thus be defined as the art of language, as distinguished from fiction which is the art of written narrative, from drama, the art of theatrical narrative, or from the essay, the art of written rhetoric.
There are only two modes in which any genre can be written, prose and verse. Prose is unmetered language; verse is metered language. Any of the genres can be written in either of the modes; that is, there are prose narratives and verse narratives, prose dramas and verse dramas, prose essays and verse essays. Likewise, there are prose poems and verse poems.
There is, thus, only one logical answer to the question “What is the difference between poetry and prose?” Poetry is a genre, and prose is a mode.
–from pages 4-5 of The New Book of Forms: A Handbook of Poetics (1986) by Lewis Turco