There are many types of poems, from free-form verse to the epic. But one of the most recognizable and for centuries often-used forms is the Sonnet. A sonnet is a poem of 14 lines. In English, the lines are in iambic pentameter, meaning that each line is 5 metrical feet long (or about 10 syllables), and each foot is an iamb, or a pattern of one unstressed and one stressed syllable. The Sonnet also has several conventional rhyme schemes, including two described as the "Italian" (or "Petrarchan") and "English" sonnet.
The Italian sonnet usually has an abba, abba octave (that is, set of 8 lines) followed by a sestet (set of 6 lines) with a rhyme pattern cde, cde (or cdc, cdc). In this type of sonnet introduces a proposition or problem in the first 8 lines and then describes a resolution in the last 6.
The English sonnet was an innovation by English poets on this Italian poetic form. Thomas Wyatt introduced the sonnet to England in the early 16th century, and his sonnets were often translations or adaptations of the work of Petrarch. Soon, though, other English poets like Philip Sidney and William Shakespeare developed a distinctly English version of this form. In the English sonnet, the usual rhyme scheme is abab, cdcd, efef, gg. The final couplet (2 lines) represent a turn (often unexpected) in theme or imagery.