Mindless of its just honours; with this key
Shakespeare unlocked his heart; the melody
Of this small lute gave ease to Petrarch's wound ...
A sonnet is a structured poetry form with origins in medieval Europe. The exact structure varies from author to author, but there are two main types: Italian, AKA Petrarchan, and English, AKA Shakespearean. The forms differ in their rhyme schemes and division of stanzas, but they share these defining attributes:
- Fourteen lines.
- Iambic pentameter (ten syllables per line, with an unstressed-stressed pattern).
- A reflective or introspective tone, often romantic.
The English sonnet consists of three quatrains (sets of four lines), each with an ABAB rhyme scheme, and a rhyming couplet which generally serves to reinforce or subvert what the rest of the poem has said. The Italian sonnet comprises an octave (eight lines), rhyming ABBAABBA, and a sestet (six lines), rhyming CDCDCD or CDECDE.
As always, The Other Wiki has more information, and examples abound in the public domain.