Arising out of Italian love poetry, sonnets eventually evolved to the point where the first part (or parts) posed a question, introduced a problem or presented some sort of emotional tension. The remainder of the poem would answer the question, solve the problem or resolve the tension. Sonnets do all this in only 14 lines. There have been experiments with rhyme scheme and other aspects of the sonnet but the 14-line dictum remains untouched.
The Italian, or Petrarchan, sonnet has two sections, one of eight lines and one of six. The eight-line section is actually two rhymed quatrains (four lines) each of which has the scheme of "a-b-b-a". Rhyming in the six-line section can vary but most often is "c-d-e" for each set of three lines.
Brought into the English culture, the sonnet underwent several changes. From the previous use of two sections to propose and resolve situations or emotions, the English sonnet divided into three separate quatrains and a couplet. The three quatrains each had an independent rhyme scheme, typically: "a-b-a-b, c-d-c-d, e-f-e-f" and "g-g" for the couplet. This was the form propagated by William Shakespeare and others in the 16th Century. Another English form, Spenserian, kept the three quatrains and couplet but substituted a linked rhyme scheme between the quatrains: "a-b-a-b, b-c-b-c, c-d-c-d and e-e." Notice how the second and fourth rhyme carries over to the next quatrain, stopping with the third.
Another formal aspect of the sonnet is its meter. Again, experimentation has occurred but the constant meter has been the use of iambic pentameter. This is a measure of five poetic feet (10 syllables) with the accent on every even beat (iambs); e.g., I THINK that HE will RUE the DAY he LIED.