Now we're just showing off . . .
Ah, sonnets. Little 14-lined gems. Intricately rhymed, concise and frequently highly imagistic invocations. But more discursive than the haiku - the form of the sonnet lends itself to a structured argument of assertions and counter assertions, the shape of which can be dictated by the form itself.
Though there are many variations on the sonnet, most follow the pattern of either the 'Shakespearean' (aka 'English') style, or the 'Petrarchan' (aka 'Italian') style. Interestingly, Neither Shakespeare nor Petrarch invented the sonnet, or the variation named after each. Surrey (a predecessor of Shakespeare) initiated the English form, and I forget who started the Italian one. Some Italian guy, I imagine.
Anyway, the two forms refer to the 'rhyme scheme' employed in the poem - rhyme scheme meaning the pattern of rhyming words at the end of each line. For example, the pattern Laura refers to - a,b,a,b c,d,c,d e,f,e,f, g,g - describes which lines rhyme with which lines in the poem. 'a' refers to the first rhyme sound used (whatever it may be). As you can see, it is repeated in the 1st and 3rd lines. 'b' refers to the next rhyme sound; it's used in the 2nd and 4th lines.
So who cares? - you ask. Well, I do, because I'm a freak, but I think you should, too, for the following reason. The rhyme sounds typically aren't merely ornamentation. Ultimately, they work to structure the entire poem, breaking it up into units that the poet uses to guide you through his or her train of thought.
For example, in the Shakespearean scheme described above, the first two rhyme sounds create a 4-line unit. Unified by the interlocking sounds of the a and b rhymes, the first 4 lines are set off - as a unit of sound, and ultimately, a unit of thought. The same can be argued for c,d,c,d and e,f,e,f. Now g,g is interesting, because it is a couplet. It's not an alternating rhyme, so it provides a sort of cap (both in terms of sound and in terms of idea) at the end of the sonnet.
As a result, Shakespearen sonnets tend to break down into the following structure. The first quatrain (the first 4 lines) proposes some idea. The second quatrain gives an example. The third quatrain gives another example, or makes a counter argument. The last couplet sums up. See - the ideas (the stages in the thought process) are marked off by the pattern of the rhymes. Nifty.
Now with the Italian form, you get a similar thing happening, but because the rhyme scheme is different, the argumentative structure differs as well. In the Italian sonnet, the rhyme scheme is as follows: