What are the characteristics of Shakespearean sonnets?

The Shakespearean or English sonnet is a form that includes a fixed meter and a typical structure. 

The Shakespearean sonnet consists of fourteen lines. It is divided into three open quatrains followed by a couplet. The quatrains are rhymed ABAB CDCD EFEF. The following couple is rhymed GG. Although most English sonnets are written in iambic pentameter, Shakespeare's Sonnet 145 is written in iambic tetrameter, and it is also possible to use alexandrines (iambic hexameter lines) for the couplet. 

Typically, each quatrain has an individual theme or focus and the couplet expresses an unusual, unexpected, or paradoxical twist, although sometimes an English sonnet can have a Petrarchan turn after the first two quatrains and a second turn or counterturn before the couplet. 

In terms of subject matter, Shakespeare's sonnets are typical of the genre of the whole in his period in focusing mainly on love or close friendship. Later poets expanded the variety of themes addressed in sonnet form.