The similarity is that they both have 14 lines. The differences are mostly regarding form and rhyme. Namely, Petrarchan sonnet has 14 lines, where the first 8 lines (or an "octave) has the rhyme scheme ABBA ABBA, and the last 6 lines (or a sestet) has a varying rhyme scheme, but usually CDECDE or CDC CDC. On the other hand, a Shakespearean sonnet also has 14 lines, which are divided into three quatrains (4 lines) and one couplet (2 lines). The rhyme scheme in the quatrains is ABAB CDCD EFEF, and the final couplet GG.
