How Are Shakespeare's Sonnets Different From The Classical Form?

The classic or Petrarchan sonnet consists of two short section of eight and six lines, with a change of idea after the eighth. The Shakespearean or English sonnet also has 14 lines in total, but is divided into three quatrains (four-line poems) and a final couplet, or pair of rhyming lines. The rhyme scheme is usually abab, cdcd, efef, gg.
The effect of this is to create a different mood from that of a Petrarchan sonnet. The Shakespearean version typically leads up to the final couplet and ends with a summary or wry comment on the poem. One example is "An expense of spirit in a waste of shame," which describes the blind fury of lust and the bleak disappointment once it is satisfied. The final couplet looks back at the description and ruefully admits, "All this the world well knows, yet none knows well/ To shun the heaven that leads men to this hell."