Popularized by the 14th-century Italian poet Petrarch, who established the custom of presenting a problem, situation, or incident in the octave, followed by a resolution in the sestet. In Petrarch's work, these were usually problems, situations, and incidents arising from his love for the unattainable Laura. When English poets began imitating Petrarch's sonnets in the early 16th century, they continued this thematic focus on the pleasures and frustrations of love. But English poets eventually developed a more flexible sonnet form which could be divided into octave and sestet, in the manner of Petrarch, or into three quatrain-length variations on a theme followed by an epigrammatic couplet. More information about the Petrarchan and English sonnet forms (and other varients) is provided in "Poetic Form: Sonnet" via the EDSITEment-reviewed Academy of American Poets.
For practice with the two popular forms, use the EDSITEment Parts of a Sonnet worksheet to test your knowledge [PDF - Acrobat reader required].