A villanelle is a 19-line poem with a fixed form, including two repeated rhymes and two refrains. If you memorize a villanelle and recite it in class, your English teacher will be very impressed!
The villanelle got its start as a poetic ballad influenced by a rustic Italian song called a villanella. Though the form has evolved, it still includes song-like refrains, giving the poem a musical sound. A villanelle has five stanzas of tercets, or three lines, and one quatrain (four lines). One of the most well-known villanelles in English is Dylan Thomas's "Do not go gentle into that good night."