Sonnets are a type of poem that originated in Italy and were popularized by William Shakespeare and Edmund Spenser. They are composed of fourteen lines, usually in iambic pentameter, and have a specific rhyme scheme. Shakespearean sonnets typically follow the rhyme scheme ABAB CDCD EFEF GG, while Spenserian sonnets follow the rhyme scheme ABAB BCBC CDCD EE. Both types of sonnets often explore themes of love, nature, and mortality.