A sonnet is poem written in 14 lines. These lines can be broken down into four quatrains. Sonnets are also written in iambic pentameter, a language scheme that has 10 beats per line with each line having alternating stressed and unstressed syllables.
The first three quatrains features four lines each that rhyme in an alternating manner. The fourth quatrain has two lines that both rhyme. A sonnet may also have a very rigid rhyming scheme that is present throughout the work where every other word in a line rhymes. For example, Shakespeare's sonnets have an ABAB / CDCD / EFEF / GG rhyming scheme.