Sonnets speak of love and strife, and they usually do so in three types or styles: Shakespearean, Petrarchan and Spenserian. All are 14 lines long and written in iambic pentameter, with five beats to the line, but sonnets are otherwise different in their characteristic forms and styles. Because they're named after their creators, the voice of each author in his respective sonnet makes a great deal of difference.
Shakespeare and the Single Idea
The Shakespearean or Elizabethan sonnet divides into three quatrains -- four-line stanzas -- and ends with a couplet, a two-line stanza. The rhyme scheme of this type is abab cdcd efef gg. Typically, a Shakespearean sonnet explores a single idea, with a third-stanza shift that will either lighten or darken the mood. Shakespeare and his contemporaries added ironic twists and contradictory touches or expanded a final, overwhelming point. One example is "My Mistress's Eyes/Sonnet 130," one of the "Dark Lady" sonnets, which criticizes the lady's features but ends by declaring her glorious in her entirety.
Petrarch and the Answered Questions
The Petrarchan or Italian sonnet maintains a consistent rhyme scheme in its first octave or eight-line stanza, which sounds out as abbaabba. After that, the remaining sestet six-line stanza goes all over the place: cddcdc, cdcedc, cdecde or cdcdcd. The Petrarchan sonnet, even more than the Shakespearean, shifts entirely at its break: The octave is a question, the sestet an answer. Milton's "When I Consider How My Light Is Spent" agonizes over God's decision to destroy his writing career with blindness in the octave; by the sestet, the author accepts the situation gracefully.
Spenser and the Pyramid
The Spenserian sonnet is most like the Shakespearean, but Spenser uses the rhyme scheme of abab bcbc cdcd ee. In other words, the Spenserian sonnet builds its successive rhymes on those of previous lines so that all of his poetry arises, pyramid-like, to heights that his beginning stanzas founded. The scaffolding style of this particular type is appropriate, as Spenser himself declared his intention to rise from expositions of human love to higher discussions of philosophy and virtue; his work "The Faerie Queene" exemplifies that desire for higher ground in love and life.
Voice and Content
Shakespeare's directly practical voice in his sonnets contrasts sharply with Petrarch's equivocation as he attempts to weigh answers and Spenser's loftiness as he tries to rise above humanity. However, the sonnet creators built on one another's work. Petrarch, who lived two centuries before Shakespeare, influenced the bard in his romantic content, as both authors wrestled with the idea of Platonic love pitted against fleshly passion. Spenser's altruism might be in response to this struggle, as he sought to rise above the emotional fray that love builds in the heart.