To understand the structure of a sonnet it is necessary to understand the definition of common terms involved with them. Many sonnets are written in iambic pentameter. Simply put, an iamb is a variety of the rhythmic unit, called the foot, and consists of a single unstressed syllable that is followed by a stressed one. In this context, pentameter means repeating this iamb five times. A quatrain is four lines of a poem, also known as a stanza. And a rhyming couplet is simply two lines together that must rhyme.
The Shakespearean, or English, sonnet consists of 14 lines each with 10 syllables. It is also written in iambic pentameter. It begins with three quatrains and ends with a rhyming couplet. The basic rhyming structure can be expressed "ABAB CDCD EFEF GG," in that "A" rhymes with "A" and so on. The three quatrains have different rhyme structures followed by a rhyming couplet. The three quatrains are usually used as a distinct structure in themselves with each being used to express a singular idea or concept.
The English sonnet is a way of expressing an idea, usually a problem or a question, yet they are designed to also have a resolution, a solution or an ending that provides a new perspective. In this way the first quatrain is used to introduce the problem or question. The second quatrain expands on the theme and can add new ideas. The third quatrain marks a twist where the problem or proposition begins to be questioned. The third quatrain usually begins with a "but." And the rhyming couplet at the end should resolve everything and leave the reader with a new discovery or perspective.
The Italian, or Petrarchan, sonnet is also 14 lines, but is structured quite differently. It consists of eight lines at the beginning (two quatrains) and ends with a closing minor group of 6 lines known as the sestet. The rhyming structure can be expressed as "ABBA ABBA CDE CDE." In this structure the octave usually represents – or poses -- a problem, and the sestet is the resolution. The ninth line is known as the turn, or volta, and marks the distinct point in the sonnet where the tone or nature changes.