Length, form, and function. Novels are longer, usually arranged in chapters, and they serve a different purpose than poetry. Poetry is meant to convey it's message through the lyrical use of words and language techniques. There are a few different ways to write and organize novels, but poetry comes in a variety of forms structured, non-structured, rhyming, non-rhyming, and etc. I'm wondering what has prompted this question.
A poem is developed around a controlling metaphor and usually has a singleness of purpose. Its structure differs greatly from a novel, for it does not always have a sequence of events or a development of a character or characters. Often a poem is limited to feeling or a single impression. Poetry is the language of feeling; novels may be expressions of abstract concepts rather than feelings.
The main difference between the poem and the novel is the care in which words are selected. They are selected as much for their sound as for their meaning. The meaning, though, is to portray experience in a heightened, concise way, and most often does not develop an intricate plot or flesh out the characters. We appreciate poetry for the intensity of feeling, the insights, and the quotable lines.
Similar to a short story, a poem is ultimately the attempt to capture a single moment/focus on a particular experience; there is only room for a single arc, and this limits and sharpens focus. The words of a poem must be chosen with extreme care, as the author is not just representing, but distilling -- sloughing away detritus until the essence of the subject remains. In a novel, there is time for many moments and many experiences. An author can tackle a more complex arc, or a series of arcs, and the overall impression/effect becomes more important than the line.
Of course the obvious difference is length--although Homer's The Iliad and Dante's Inferno and Cervantes' Don Quixote are all long works in verse form. Let's assume we're not talking about these kinds of works for the sake of this comparison.
Another difference is development of plot. A poem may or may not have a narrative (tell a story), while a novel always does. It has characters and places and happenings, which a poem may or may not have.
Poetry is generally, as the post above suggests, created by using figurative and/or rhythmical language. Novels generally have those elements, as well, but they are diffused throughout the novel rather than condensed into something more compact.
While the message of a poem and a novel may be the same, the form clearly is not. Poetry is compressed and compacted; novels are elongated and elaborate. For me, though, a novel generally provides an opportunity to have a relationship with characters (and learn more about myself) as they live the lives the authors have given them, while a poem generally invites me to sink into an image or an idea or a feeling which also gives me an opportunity to learn more about myself and my connection to the world.
This was more difficult than it sounds, and I'm not sure I really said what I meant--but there it is.
A poem has a structure. It usually has verses, rhymes, strophes, a rhyme scheme and a rhythm.
The words are suitable and the order that words have in a poem is well-chosen.
There are figures of speech that are not easy to understand because there isn't an easy explanation. It could be possible that you have to analize the poem in a deeper way. It usually doesn't happen when you read a novel.
In a novel there is an omniscient author(third-person) that let you clearly understand what's going on during your reading.
It could also occur that the narrator is a first-person narrator. In this case you will discover the story together with one of the characters focusing on his/her feelings during his/her journey.
A poem is a written "acting-out" of something that the author wants to say in a particular elegant way, sometimes it could even be used to overcome a censorship imposed by a government.
Novel is a story where as poem is like a song that might or might not rhyme and include various literary devices.