Petrarch (1303 - 1374), a scholar and poet of early Renaissance Italy, did not invent the sonnet form, but it is his name which is associated with the form before Shakespeare adopted it and made it his own. The nature of the Italian language makes rhyming much eaier than in English. The sonnet was introduced in England by Thomas Wyatt (1503-1564). Henry Howard (1516-1547 executed for treason) is said to have given the sonnet its rhyme scheme (abab cdcd efef gg) and its characteristic division into three quatrains with a concluding couplet.

Almost all of Shakespeare’s sonnets conform to this structure. the stylistic form of the English sonnet—the rhyme scheme, the 14 lines, and the metre. Shakespeare’s, however, subverts the genre: instead of "expressing worshipful love for an almost goddess-like yet unobtainable female love-object, as Petrarch, Dante, and Philip Sidney had done, Shakespeare introduces a young man. He also introduces the Dark Lady, who is no goddess. Shakespeare explores themes such as lust, homoeroticism, misogyny, infidelity, and acrimony in ways that may challenge, but which also open new terrain for the sonnet form". [Good old wikipedia puts it quite well.]