The "sonnet" is traced back to Francis Petrarch, whose sonnets (14th C Italian) were popular throughout western Europe. Early sonnets were less strict about form, and a sonnet could be slightly longer or shorter than the traditional 14 lines of rhymed iambic pentameter. Because Petrarch's sonnet sequence (over three hundred poems!) traced the course of an unhappy love, sonnets tended to be written from an unsatisfied male lover to a not-forthcoming female beloved.
The "Petrarchan" or "Italian" sonnet is usually
14 lines, iambic pentameter, divided into an 8 line octave rhymed abba abba and a 6 line sestet rhymed with some combination of "c" and "d" and/or "e" such as cddcee or cdcdee or cdecde or cdcdcd etc.
The two parts of the Italian sonnet always worked together in some way, usually with some degree of contrast. The octave could ask a question that the sestet answered, or the octave could be "his version" and the sestet "her version" of what went wrong, or the octave could be what the lover loved about the beloved, and the sestet what he hated about her, etc.
The Italian sonnet develops a near-schizophrenia in its interest in topics that have two sides, but it works particularly well for Renaissance love affairs since Renaissance lovers often felt conflicted about their feelings for another person (especially in a poetry that often celebrated illicit love).
The Italian sonnet form did not work well in English because many fewer English words end in tense vowels and rhyme with one another. William Shakespeare's sonnet rhyme form becomes popular with many other English language poets -- abab cdcd efef gg. This rhyme does not lend itself to a balanced two part form, and Shakespeare's sonnets tend to have an unbalanced two part form. The first part is OFTEN the first 12 lines, subdivided into three little bits -- three images that complement one another, three reasons why the speaker loves the beloved, three parts of the beloved's body that the speaker dwells upon, three metaphors that describe the speaker's feelings, etc. The unbalance comes about because the last two lines -- the "gg" couplet have to pack quite a punch in order to carry half the meaning, and many Shakespearian sonnets have very clever couplets that undercut, deny, shed a new light upon, or contrast the first 12 lines.