Form is poetry’s container. Form can define a poem as a sonnet, sestina or villanelle, for example.  Form can be to a poem as framing is to a house, defining shape and size and in the end providing spaces on which to hang your pretty pictures and shelves of stored knowledge, or in which to arrange the furniture for your scenes of comedy or drama. Any good decorator knows that the character of their work is defined by the space. So it often is with poetry.