There are generally two major kinds of sonnets. The Italian, also known as the Petrachan, was named for Francesco Petrarca, who mastered the form in Italy. The Petrarchan sonnet:
...is a verse form that typically refers to a concept of unattainable love. It was first developed by the Italian humanist and writer, Francesco Petrarca. Conventionally Petrarchan sonnets depict the addressed lady in hyperbolic [exaggerated] terms and present her as a model of perfection and inspiration.
The second sonnet form is the Shakespearean. (Shakespeare did not come up with the form: actually Sir Thomas Wyatt is usually credited with introducing the sonnet to England—from Italy, but it is named after Shakespeare perhaps because he wrote so many [beautiful] sonnets: 154 of them.)
The Italian sonnet has some very distinct similarities to the Shakespearean sonnet (in that it came first, this is no surprise).
The Italian sonnet has fourteen lines, written in iambic pentameter, which means that there are ten syllables per line, with stress on every other syllable. There are two parts to the Italian sonnet form: the octave (made up of the first eight lines of the sonnet) and the sestet (made up of the last six lines of the sonnet). Poets will often use the parts of a sonnet to organize their ideas.
The rhyme scheme is a pattern of end-rhyme that the poet uses, where the first and third line rhyme on the last word, and the second and fourth line rhyme on the last word, and so forth. Whereas the Shakespearean sonnet has only one rhyme scheme, the Italian sonnet has several options. The rhyme scheme for the first eight lines (the octave) is generally consistent: abba abba. However, for the last six lines, there are a variety of choices...
including: c d d c d d, c d e c d e, c d d e c e, c d c d c d, c d d c e e, c d c d e e, c d d c c d
The word "sonnet" comes from the Italian word, "sonnetto," which means "little song."
The Shakespearean sonnet is also known as the Elizabethan sonnet. It also has fourteen lines and is written in iambic pentameter. However, its sections are different. The first twelve lines consist of three quatrains—a quatrain is a four-line stanza. The last two lines are called the rhyming couplet: this is a pair of lines that rhyme.
The rhyme scheme of the Shakespearean sonnet is:
abab cdcd efef gg
Shakespeare often used the "segments" (or parts) of this form to organize his ideas. For example, in Sonnet 29, Shakespeare spends the first eight lines (two quatrains) talking about how awful his life is. The first word of the ninth line is "yet," which is a pivotal word, where the entire tone of the poem changes—at this point, the author remembers the woman he loves (to whom he is writing the sonnet), and when he thinks of how wonderful she is and remembers how glorious she makes him feel, he announces (in the rhyming couplet, that serves as a conclusion) that he would not change places with kings because his life is so extraordinary with her.
(There is also one minor sonnet form named for Edmund Spenser, who wrote "The Faerie Queene," dedicated to Queen Elizabeth I. It was invented by Spenser for the above noted epic poem, and follows very different structural guides.)
Shakespeare's sonnets are quite famous, but he is certainly not the only poet to use this sonnet form. The topic of most Shakespearean (Elizabethan) sonnets is "love."