Structure (poetry) - The pattern of organization of a poem. For example, a Shakespearean sonnet is a 14-line poem written in iambic pentameter. Because the sonnet is strictly constrained, it is considered a closed or fixed form. An open or free form poem has looser form, or perhaps one of the author’s invention, but it is important to remember that these poems are not necessarily formless.
Symbolism - when an object is meant to be representative of something or an idea greater than the object itself.