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.