The sestina is a type of formal poem that relies on an exact structure of language. A sestina, by definition, consists of 39 lines — six six-line stanzas, followed by a three-line stanza, known as a tercet. Traditionally, the sestina was written in iambic pentameter; however, most writers today do not see that as a necessity for the form.
In a sestina, the writer must choose six different words which will be used as the last word in each line of the poem. For example, the last word for each line in the first stanza could be: dog, rain, child, sin, garden, fight. In the next stanza, the words shift in order; the second stanza's lines would then end: rain, child, sin, garden, fight, dog. This process continues until the sixth stanza.
In the final tercet, the first line will contain the words "dog" and "rain", the second will contain "child" and "sin", and the third will contain "garden" and "fight"; however, they need not be at the end of the line.
The sestina was first invented in the 12th century, by a famous French troubadour, Arnaut Daniel. Throughout history, many well-known poets have written sestinas; among them, Elizabeth Bishop, Dante, Ezra Pound, and W.H. Auden. Additionally, a popular online literary journal, McSweeney's Internet Tendency, regularly publishes sestinas on various subjects — a recent submission is entitled, "Sestina with Clementines, Beer, and Guitar."
Because the sestina has such a strict form, many writers find that writing sestinas help them to explore their creativity. Because they are limited to reusing certain words, they are forced to come up with original lines where the words will fit in. Often, writers will start a poem as a sestina, and then edit it down into a less structured free-verse poem.
If you want to write a sestina, the best way to learn is to study examples of sestinas by other writers. Some successful, well-known sestinas that are readily available are Elizabeth Bishop's "Sestina" and Ezra Pound's "Sestina: Altaforte." Many books on poetry also focus on sestinas; for a good overview of the sestina and other forms of poetry, read Mary Oliver's A Poetry Handbook.