A Sonnet is a poem consisting of 14 lines in iambic pentameter with a particular rhyming scheme. A Shakespearean sonnet has three quatrains and a couplet, and rhymes abab cdcd efef gg.
An Petrarchan/Italian sonnet is composed of an octave, rhyming abbaabba, and a sestet, rhyming cdecde or cdcdcd, or in some variant pattern, but with no closing couplet. Usually, English and Italian Sonnets have 10 syllables per line, but Italian Sonnets can also have 11 syllables per line. There are many variations (See https://en.wikipedia.org/wiki/Sonnet)