What is the difference between prose and poetry?

Long story short.

The difference is that with PROSE the rhythms varies more than poetry.

The different beats and rhythms between POETRY and PROSE can easily tell them apart

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Poetry is a sample of literature written in verse or meter. It may or may not rhyme. It may or may not consist of stanzas. It can be sung, chanted, recited, spoken or written according to some measure or pattern of recurrence. It emphasizes the importance of sounds of words, not only meanings.

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Formerly, poetry could be distinguished from prose by the musical sound of it when recited or read aloud. Poetry used to be constructed according to forms and conventions such as rhyme and rhythm ( and in Classical languages, the play of long and short syllables ) and also it allowed a more flexible treatment of word order and grammar than prose allows.

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Prose is distinguished from poetry by its greater variety of rhythm and its closer resemblance to the patterns of day to day speech. The word prose comes from the Latin prosa, meaning straightforward, hence the term "prosaic," which is often seen as pejorative. Prose describes the type of writing that prose embodies, unadorned with obvious stylistic devices. Prose writing is usually adopted for the description of facts or the discussion of whatever one's thoughts are, incorporated in free flowing speech.

Poetry (from the Greek "poiesis," a "making" or "creating") is a form of art in which language is used for its aesthetic and evocative qualities in addition to, or in lieu of, its ostensible meaning. Poetry may be written independently, as discrete poems, or may occur in conjunction with other arts, as in poetic drama, hymns or lyrics.

Poetry is all about conveying meaning without saying as much. It alludes rather than proclaim, it hints rather than describe. Suggestiveness is its soul. Poetry is like drawing a picture, a vivid imagery with words. Prose is like using a series of pictures to tell a story, or a story within a story. Prose can be lyrical, but for poetry that's an essential quality.

*Edit (englishlearning): Read more about this difference in 'Manipulate Your Marker: A Proactive Approach to Success in the HSC' - find it at Dymocks and Angus & Robertson.
Prose is a form of writing that is used daily while poetry is an artistic form of writing. Prose is straightforward while poetry may contain rhymes or be decorated.