The Italian term sonetto simply means a short or little song. The fourteen-line sonnet form with which later became known as the "Italian" or "Petrarchan" sonnet appears to have first been regularized in the work of Guittone of Arezzo, a thirteenth century Italian poet. Petrarch (1304 – 1374) employed the form in his 366-sonnet sequence, Rime Sparse, a collection of poems on the subject of his love for a woman named Laura. Sir Thomas Wyatt (1503 – 1542) and Henry Howard, Earl of Surrey (1517 - 1547) translated Petrarch's sonnets into English and are generally regarded as the founders of the English sonnet tradition. Wyatt, in particular, used the Petrarchan sonnet form extensively in his own poetic works as well as his translations. Although Surrey and other poets of his period wrote English sonnets, due to the fame and influence of Shakespeare's work, the English sonnet is frequently called a "Shakespearean" sonnet.
Both forms are fourteen lines long and normally written in iambic pentameter in English. They differ in both rhyme scheme and structure.
The Italian sonnet consists of an eight-line octave followed by a six-line sestet. The octave is rhymed as two closed quatrains, ABBAABBA. The sestet has a more flexible rhyme scheme, using two or three rhyme sounds, with common possibilities being CDECDE or CDCDCD. Its main structural feature is a "turn" or shift of focus between the octave and sestet.
The Shakespearean sonnet consists of three open quatrains followed by a couplet, i.e. its rhyme scheme is ABAB CDCD EFEF GG. In a typical English or Shakespearean sonnet, each quatrain has an individual theme, and then there is a break before the couplet, which can then either summarize the sonnet or work as a sort of surprise ending. Sometimes there is a minor turn after the second quatrain as well, but this is not an essential feature of the form.