A villanelle is A. a formal poem using extensive repetition.
It is a nineteen line poem which has five tercets followed by a quatrain. Tercet meaning 3 lines, quatrain meaning 4 lines.
It contains two refrains and two repeating rhymes. Where the first and third line is the first tercet is repeated alternately throughout the poem.
Examples of a villanelle are:
Do Not Go Gentle Into That Good Night by Dylan Thomas
One Art by Elizabeth Bishop
The Home on the Hill by Edward Arlington Robinson