The sonnet form of poetry enjoyed two great eras of productivity, first in Italy and then in England. Both forms use iambic pentameter, which gives a pattern of ten syllable lines with an accent on every second syllable.
The Italian Sonnet
The Italian sonnet is sometimes called the Petrarchan sonnet after [Francesco Petrarch]. It is fourteen lines consisting of an octet and a sestet. The octet may have the rhyming scheme abbaabba or abbacddc. Rarely it appears as abababab. The rhyming scheme of the sestet is more flexible, but common variations are xyxyxy and xyzxyz.
In this form it is usual for the poet to create two differing moods in the octet and sestet. One common method is for the octet to state a conundrum and the sestet to give the resolution.
The English Sonnet
The Petrarchan Sonnet was adopted into England, but the relative difficulty of finding rhyming words in English meant that the form soon evolved into a more amenable form. The standard English sonnet, or Shakesperean sonnet, consists of three quatrains and a rhyming couplet. The rhyming scheme is abab cdcd efef gg.
The English poets were generally free with how they structured the mood of their piece, but the rhyming couplet at the end made for many a pithy conclusion. Famous Englishmen to utilise the sonnet include Samuel Daniel, Phillip Sydney, Edmund Spencer and of course William Shakespeare.