The sestina is a strict ordered form of poetry, dating back to twelfth century French troubadours. It consists of six six-line stanzas followed by a three-line envoy. Rather than use a rhyme scheme, the six ending words of the first stanza are repeated as the ending words of the other five stanzas in a set pattern. The envoy uses two of the ending words per line, again in a set pattern.
The sestina scheme is shown here.
The examples of sestinas I've found all seem to disagree on how many syllables they should be per line or even if the syllables per line should be consistent. When it's strict, iambic pentameter seems to be the norm.