Although Giacomo de Lentino is considered the inventor of the Italian sonnet form, dating all the way back to the Roman Emperor Frederick II (1208-1250), the form was perfected by Francesco Petrarca, also known as Petrarch, a century later (1304-1374). When we think of the traditional sonnet, we think of Petrarca's sonnets, which were written much earlier than Shakespeare's.
When comparing and contrasting Shakespeare's Sonnet 18 with one of Petrarca's, it should be noted that the meters of both are still in iambic pentameter and both contain 14 lines, but the biggest difference is the rhyme scheme. Shakespeare made use of the abab cdcd efefgg rhyme scheme that was created by the Earl of Spencer (approximately 1517-1547) several decades before Shakespeare might have started his sonnets (approximately 1593). Petrarca, on the other hand, created the abba abba cdecde pattern and sometimes ended with cdcdcd.
If we compare Shakespeare's Sonnet 18 to Petrarca's Sonnet 1, we also see a difference in content. Sonnet 18 is very clearly about someone the speaker is in love with. Shakespeare compares his love to summer imagery because summer fades, but he wants to argue that his love shall not fade. While Petrarca's sonnet also speaks of his love life as a youth, as seen in the translated line, "I fed my heart / in my first vagrant youthfulness," Petrarca's sonnet is more about spirituality and remorse. Petrarca's sonnet says that he hopes "to find pity, and forgiveness." He also speaks of being "ashamed."