This is a classic Shakespearean sonnet with fourteen lines in very regular iambic pentameter. With the exception of a couple relatively strong first syllables (and even these are debatable), there are basically no deviations from the meter. There aren’t even any lines that flow over into the next line – every single line is end-stopped. There are two quatrains (groups of four lines), followed by a third quatrain in which the tone of the poem shifts a bit, which is in turn followed by a rhyming couplet (two lines) that wraps the poem up. The rhyme scheme is ABAB CDCD EFEF GG.
The form of this sonnet is also notable for being a perfect model of the Shakespearean sonnet form. Just as in older Italian sonnets by which the English sonnets (later to be called Shakespearean sonnets) were inspired, the ninth line introduces a significant change in tone or position. Here Shakespeare switches from bashing the summer to describing the immortality of his beloved. This poem also has the uniquely English twist of a concluding rhyming couplet that partially sums up and partially redefines what came before it. In this case, the closing lines have the feel of a cute little poem of their own, making it clear that the poet’s abilities were the subject of this poem all along.
Don’t be fooled, though: beyond the form, this is not your stereotypical sonnet. The main reason is that sonnets, at least before Shakespeare was writing, were almost exclusively love poems. Certainly this poem has some of the qualities of a love poem, but, to say the least, this poem isn’t just a poet’s outpouring of love for someone else. Check out the "Love" theme for more on that.