There are a lot of sonnets floating around in the poetic ocean, so make sure you've got hold of the right fish when you start talking about form. Shakespeare's sonnets follow the form known as—wait for it—Shakespearean sonnets. They're also called "English sonnets," just so everyone else doesn't feel left out. After all, Shakespeare doesn't even get credit for inventing this baby. Petrarch, an Italian who wrote some pretty major stuff in the fourteenth century, invented this 14-line poem, but a trio of pre-Shakespearean English dudes (Thomas Wyatt, the Earl of Surrey, and Philip Sidney, if you're interested) developed it into the form Will uses: 3 quatrains plus a couplet, with an overall ABAB CDCD EFEF GG rhyme scheme.
But let's rewind to some basics: what the kerfuddled munchkin is a quatrain? It's actually pretty simple: just a stanza of four lines, usually with alternating rhymes. A couplet, which even sounds like "couple," is a group of two lines that rhyme. Check out the ending of Sonnet 55:
So, till the judgment that yourself arise,
You live in this, and dwell in lovers' eyes (13-14)
The rest of the poem rhymes every alternate line, but this final couplet ties it all together with a satisfying sound repetition. It's Shakespeare's way of saying, Thaaaaaaat's all, folks.
The other sonnet-only feature you should keep in mind is the volta, which means "turn" in Italian (think "revolve"). It refers to a major thematic shift in the sonnet that usually occurs at line 9. After the first two quatrains have laid it out, the volta flips things around, signaling a new argument, a fresh perspective, a wise conclusion, or a change in mood.
In Sonnet 55 the volta isn't so dramatic, but line 9 does shift the poem away from images of war and decay to active descriptions of the beloved's triumph over time and death. We always knew that poetry was winning, but the final quatrain makes it clear: this sonnet's gonna keep killing it till the end of time.
These sonnets rock their iambic pentameter, by far the most popular meter in Elizabethan England. That means each line has 10 syllables with a stress on alternate syllables:
The living record of your memory (8)
The two-syllable combination of unstressed and then stressed forms an iamb (it sounds like daDUM). So if each line has 10 syllables total, that makes—hold on while we punch it out on our calculators—five iambs per line (producing this beat: daDUM daDUM daDUM daDUM daDUM). Iambs: putting the "pent" in "pentameter" for five hundred years.
In Elizabethan England, any poet who wanted to go prime-time wrote a sonnet sequence: basically a bunch of semi-related sonnets grouped under a single title. Following Petrarch, the grand-daddy of sonnet sequences, these poems are mostly about love and ladies (and sometimes men). All the cool kids were doing it, like Philip Sidney ("Astrophil and Stella") and Edmund Spenser ("Amoretti").
Shakespeare got in on the game too, but he gave his sonnets even less of a narrative than some of his contemporary pals. There's not a whole lot of action in these 154. Weighty themes, beautiful imagery, and philosophical wit? Yes, yes, and yes. But we don't even know the name of the beloved.
So what does a sonnet bring to the poetic table? The emotion may be wild and the suffering intense, but the form keeps it under control. With strict iambic pentameter and alternating rhymes to hold the feelings in, this sonnet is as firm and elegant as a stuffed valentine's heart.