What is the difference between a poem and a novel?

The poem and the novel are two entirely different literary forms. According to the Merriam-Webster Dictionary, a poem is defined as a 

...piece of writing that usually has figurative language and that is written in separate lines that often have a repeated rhythm and sometimes rhyme.

Some poems are essentially narratives, telling a story, while others address a single theme or topic. Poems are usually (but not always) shorter in length than novels, which are,...

The poem and the novel are two entirely different literary forms. According to the Merriam-Webster Dictionary, a poem is defined as a 

...piece of writing that usually has figurative language and that is written in separate lines that often have a repeated rhythm and sometimes rhyme.

Some poems are essentially narratives, telling a story, while others address a single theme or topic. Poems are usually (but not always) shorter in length than novels, which are, in short, books. Aside from length, a novel always tells a story--it is fundamentally a work of fiction. A novel usually covers a certain timeframe, even if not in a linear way--it begins with an event and ends with an event. Perhaps most important, novels are written in prose form. They are not concerned with rhythm, rhyming schemes or many aspects of poetry. Many poems and novels address the same thematic issues--Wilfred Owen's poem "Dulce et Decorum Est" and Erich Maria Remarque's novel All Quiet on the Western Front each make an almost identical point about war--but they have very different stylistic and structural elements.