What is the Difference Between a Shakespearian and a Petrarchan Sonnet?

The sonnet is a poetic form first popularised by the Renaissance Italian poet Petrarch. Traditionally in English, sonnets are written in iambic pentameter and have fourteen lines. Traditionally in English, there are two forms of sonnet, the Shakespearian (popularised by William Shakespeare) and the Petrarchan or Italian. 

Because Italian is a far more rhyme-rich language than English, Petrarchan sonnets are less varied in their rhyme endings. These sonnets are usually divided into two quatrains and a final sestet. With the rhyme scheme ABBA, ABBA (or occasionally ABAB, ABAB) and CDE, CDE or CDC, DCD. The first two quatrains tend to establish a question or a status-quo which is either answered or question at the turn of the poem, or the volta, which occurrs after the eighth line, or at the final sestet. Despite the technical difficulty of using a model for a rhyme-rich language for a language which is considerably poorer in rhymes, some anglophone poets have chosen to write Petrarchan sonnets, though the Shakespearian form is more usual.

The Shakespearian sonnet is divided into three quatrains and a final couplet. Following the rhyme scheme ABAB, CDCD, EFEF, GG. The final rhyming couplet plays a crucial role to the meaning of the poem, often refuting what was established in the first three quatrains.