Sonnet
A sonnet is a poetic form which originated in Italy during the late Middle Ages/Early Renaissance, but spread out across the world over the ensuing centuries. The simplest description of the sonnet is that it is a fourteen-line poem with any of several specific rhyme schemes, usually written in iambic pentameter (that is, each line has 10 syllables that naturally pair off). It is thematically broken down into two parts, the first of which presents the subject of the poem, and the second of which responds to or reflects on that subject. The exact number lines in each part, the choice of rhyme schemes, and the number of metrical "feet" in each line varies from culture to culture, but a sonnet is always recognizably a sonnet.
While sonnets can be individual stand-alone poems, they are frequently part of larger works called "sonnet sequences", forming larger lyric or epic poetic works out of dozens of sonnets.
Over the years as it spread, the structure of the sonnet changed and mutated, branching out into several notable varieties, the most common of which are the Italian and Shakespearean. Several other known varieties (and examples thereof) can be seen by following the Wikipedia link above.
Italian (or Petrarchan) Sonnet
The sonnet was created by Giacomo da Lentini, head of the Sicilian School under Emperor Frederick II. Guittone d'Arezzo rediscovered it and brought it to Tuscany where he adapted it to his language when he founded the Neo-Sicilian School (1235–1294). He wrote almost 250 sonnets. Other Italian poets of the time, including Dante Alighieri (1265–1321) and Guido Cavalcanti (c. 1250–1300), wrote sonnets, but the most famous early sonneteer was Petrarca (known in English as Petrarch). Other fine examples were written by Michelangelo.
The structure of a typical Italian sonnet of the time included two parts that together formed a compact form of "argument". First, the octave (two quatrains), forms the "proposition", which describes a "problem", or "question", followed by a sestet (two tercets), which proposes a "resolution". Typically, the ninth line initiates what is called the "turn", or "volta", which signals the move from proposition to resolution. Even in sonnets that don't strictly follow the problem/resolution structure, the ninth line still often marks a "turn" by signaling a change in the tone, mood, or stance of the poem.
Later, the abba, abba pattern became the standard for Italian sonnets. For the sestet there were two different possibilities: cdd, cde and cdc, cdc. In time, other variants on this rhyming scheme were introduced, such as cdcdcd. Petrarch typically used an abba, abba pattern for the octave, followed by either cde, cde or cdc, cdc rhymes in the sestet.
The first known sonnets in English, written by Sir Thomas Wyatt and Henry Howard, Earl of Surrey, used the Italian scheme, as did sonnets by later English poets including John Milton, Thomas Gray, William Wordsworth and Elizabeth Barrett Browning. Early twentieth-century American poet Edna St. Vincent Millay also wrote most of her sonnets using the Italian form.
Dante's variation
Most of the sonnets in Dante's La Vita Nuova are Petrarchan. Chapter VII gives sonnet "O voi che per la via", with two sestets (AABAAB AABAAB) and two quatrains (CDDC CDDC), and Ch. VIII, "Morte villana", with two sestets (AABBBA AABBBA) and two quatrains (CDDC CDDC).
Shakespearean (or English) Sonnet
When English sonnets were introduced by Thomas Wyatt in the early 16th century, his sonnets and those of his contemporary the Earl of Surrey were chiefly translations from Italian and French. While Wyatt introduced the sonnet into English, it was Surrey who gave it a rhyming meter, and a structural division into quatrains of a kind that now characterize the typical English sonnet. The 14 lines of the English sonnet are written in iambic pentameter, and are grouped into three quatrains and a couplet, usually with the rhym scheme ABAB CDCD EFEF GG. The couplet usually represents a "turn" in the poem, either commenting on the previous lines or subverting them.
This variety of sonnet is frequently called the Shakespearean sonnet after William Shakespeare, not because he was the first to write in this form but because he became its most famous practitioner, having written 154 of them, even incorporating sonnets into some of his plays. (Romeo and Juliet, for instance, uses a sonnet for its prologue, and Romeo and Juliet's first exchange in Act One, Scene Five is one as well.)
In the 17th century, the sonnet was adapted to other purposes, with John Donne and George Herbert writing religious sonnets, and John Milton using the sonnet as a general meditative poem. Both the Shakespearean and Petrarchan rhyme schemes were popular throughout this period, as well as many variants.
Modern Sonnets
The sonnet fell out of style around the time of the Restoration, and hardly any sonnets were written between 1670 and the time of William Wordsworth. However, sonnets came back with a vengeance at the time of the French Revolution, and at the dawn of the 19th century there was an explosion of sonnets in the English literary scene. The form underwent numerous experimental mutations, with line counts varying from 10-1/2 to 24, and a much wider variety of rhyme schemes, although the traditional structures were never actually abandoned. By the end of the 19th century, the sonnet had been adapted into a general-purpose form of great flexibility.
This flexibility was extended even further in the 20th century. Among the major poets of the early Modernist period, Robert Frost, Edna St. Vincent Millay and E. E. Cummings all used the sonnet regularly, and poets such as William Butler Yeats composed major works in the form. Creative mutation of the sonnet continued through the whole of the century, with W. H. Auden not only writing traditional sonnet sequences, but also composing unrhymed sonnets as well. Half-rhymed, unrhymed, and even unmetrical sonnets have been very popular since 1950; perhaps the best works in the genre are Seamus Heaney's Glanmore Sonnets and Clearances, both of which use half rhymes, and Geoffrey Hill's mid-period sequence "An Apology for the Revival of Christian Architecture in England". The 1990s saw something of a formalist revival, however, and several traditional sonnets have been written since.