A Sonnet is a type of poem in which the structure is very specific, following a clear rhyme scheme that flowed through fourteen lines. There have been many different forms of the sonnet, generally speaking, throughout the centuries passed; However, many "sonneteers" (as they are often known) claim that the basis of a sonnet in poetry has indeed evolved throughout this time.
The word 'sonnet' is derivative from the Italian word 'sonnetto', which loosely translates into many different meanings, specifically a little poem, song, or sound. The sonnet was also created in Italy by Giacomo da Lentini, who headed the Sicilian School in Tuscany in the late 1200s. Back during that time period, many of the sonnets that were written were meant to portray the means of an argument; However, as time has gone on, we have seen the structural aspects of sonnets, as well as themes, take varying turns and changes. Sonnets have been written and published all throughout the world -- By the English, Russians, Dutch, Germans, to name a few -- and continue to be popular still today in the 21st century.
Structure and an Example
Sonnets are Lyric poems that are 14 lines and usually have one or more conventional rhyme schemes like the poem below.
Sonnet CXXX: My Mistress Eyes are Nothing Like the Sun
(a) My mistress' eyes are nothing like the sun;
(b) Coral is far more red than her lips' red:
(a) If snow be white, why then her breasts are dun;
(b) If hairs be wires, black wires grow on her head.
(c) I have seen roses damask'd, red and white,
(d) But no such roses see I in her cheeks;
(c) And in some perfumes is there more delight
(d) Than in the breath that from my mistress reeks.
(e) I love to hear her speak, yet well I know
(f) That music hath a far more pleasing sound.
(e) I grant I never saw a goddess go:
(f) My mistress, when she walks, treads on the ground.
(g) And yet, by heaven, I think my love as rare
(g) As any she belied with false compare.