What is the difference between 'form' and 'structure' in poetry?

'Form' is the type of poem the poet has choosen to write in. Sonnets, ballads, dramatic monologues and dialogues, and vilanelles are all types of form with their own particular rules and conventions: the sonnet's 14 lines, for example, or the vilanelle's rhyme scheme. 

Structure, on the other hand, is the techniques the poet is using to order the poem on the page. This might mean things like enjambment (running one line into the next, without any punctuation), lists, repetition, and caesura (breaking up a line with a full-stop or comma). The two terms are closely related, which is why you often find them together, but if you think of it like a painting -- with form as the frame and canvas, and structure as the landscape where the scene is taking place -- you can see how they work in different ways.