There are 2 types of sonnets: Elizabethan or Shakespearean, and Petrarchan or Italian. (A and B)
The Petrarchan is divided into two parts, an 8-line octave and a 6-line sestet The octave thymes abba abba, while the sestet generally thymes cde cde or uses some combination of cd rhymes. An example of this sonnet is Elizabeth Barrett Brownings "How Do I Love Thee?"
The Shakespearean sonnet has 3 four-line quatrains plus a concluding two-line couplet. the rhyme scheme of such a sonnet is usually abab cdcd efef gg. Each of the 3 quatrains usually explores a differnt variation of the main theme. At the end, the couplet (gg) presents a summarizing or concluding statement. In "Romeo and Juliet," Shakespeare incorporates two such sonnets. One is the dialogue between Romeo and Juliet when they first meet (the "pilgrim" sonnet).