What is the difference between shakespeare style of writing sonnets and that of the italian sonnets?

The term "sonnet" is derived from the Provençal word "sonnet" and "sonetto" Italian word, both meaning song. From the thirteenth century has come to signify a poem of fixed form of fourteen lines that follow a very specific rhyme scheme and a logical structure. These features have changed over time.

Italian sonnet

Italian sonnet rules set by Guittone Arezzo (1235-1294), who wrote almost 300 sonnets. Other Italian poets of his time,including  Dante Alighieri (1265-1321) and Guido Cavalcanti (c. 1250-1300) wrote sonnets, but the most famous sonetist was Francesco Petrarca (1304-1374).

In its original form , Italian sonnet is divided into an octave of eight lines (formed by two quatrains) followed by a sextet of six verses (composed of two minor thirds). Quatrains were stating a proposal or an interrogation and minor thirds were offering a solution with a clear break between the two. The eight verses were rhyming after structure a-b-b-a, a-b-b-a. At Minor thirds,  there were two possibilities for c-d-e-c-d-e or c-d-c-c-d-c. With time, other variations of rhyme began to be used. Usually the ninth line was creating a turn, a volta, which indicates a change in topic or tone.

The first sonnets in English by Sir Thomas Wyatt and Henry Howard, Earl of Surry,were using the scheme of the Italian sonnet,as well as the sonnets written by later authors as John Milton, Thomas Gray, William Wordsworth and Elizabeth Barrett Browning.

However, these poets often ignore the logical structure of the sonnet. English poets use another metric foot, which is an iambic pentameter, an equivalent of what was commonly used for Petrarca's sonnets in the Roman languages such as Italian, French, Spanish or Romanian. Another important representative of the Italian sonnet is Michelangelo:with a form sometimes less successful, his sonnets impressed by the force of expressed inner feelings.

English sonnet

Soon after the introduction of the Italian sonnet, English poets began to develop their own formula. Among them , were included Sir Philip Sidney, Michael Drayton, Samuel Daniel and William Shakespeare. This formula often has the name of Shakespeare. The structure is consisting of three stanzas of four lines (quatrains) and a couplet of two lines. Couplet usually bring a sudden change of themes or imagery. Usual rhyme scheme was a-b-a-b, c-d-c-d, e-f-e-f, g-g.