This is probably the most famous of Shakespeare’s sonnets and possibly the most famous poem in the English language. The argument is simple; the poet compares the Fair Youth to summer, arguing that he is even more beautiful than the most beautiful season.

The sonnet is interesting in that it is the first in the sequence in which the speaker doesn’t urge the young man to have his own children. The language is simple and the imagery accessible, without much of the complex wordplay or poetic devices which Shakespeare often deploys so cleverly.

Scholars speculate that the sonnet was written to a young man, part of the Fair Youth sequence of Shakespeare’s sonnets–numbers 1–126–which, along with the rest of his sonnets, was dedicated to a “Mr. W.H.” Popular candidates for the identity of W.H. include Henry Wriothesley, 3rd Earl of Southampton, and William Herbert, 3rd Earl of Pembroke, both of whom were patrons of Shakespeare.

Note that in the third quatrain, starting with “But thy eternal…”, the tone of the poem changes – from the weaknesses of summer to the greatness of the subject of the poem. This is a “volta”.

About Sonnets
A sonnet is a poem which expresses a thought or idea and develops it, often cleverly and wittily.

The sonnet genre is often, although not always, about ideals or hypothetical situations. It reaches back to the Medieval Romances, where a woman is loved and idealised by a worshipping admirer. For example, Sir Philip Sydney in the Astrophil and Stella sonnet sequence wrote in this mode. Poems were circulated within groups of educated intellectuals and they did not necessarily reflect the poet’s true emotions, but were a form of intellectual showing-off. This may not have been true of all; it is a matter of academic debate today. It is generally believed, however, that Shakespeare’s sonnets were autobiographical, although some dispute this.
BBC Podcast, Melvyn Bragg, “In Our Time” Shakespeare’s Sonnets

Sonnets are made up of fourteen lines, each being ten syllables long. Its rhymes are arranged according to one of the following schemes:

• Italian, where eight lines consisting of two quatrains make up the first section of the sonnet, called an octave. This section will explore a problem or an idea. It is followed by the next section of six lines called a sestet, that forms the ‘answer’ or a counter-view. This style of sonnet is also sometimes called a Petrarchan sonnet.

• English, which comprises three quatrains, making twelve lines in total, followed by a rhyming couplet. They too explore an idea. The ‘answer’ or resolution comes in the final couplet. Shakespeare’s sonnets follow this pattern. Edmund Spenser’s sonnets are a variant.

At the break in the sonnet — in Italian after the first eight lines, in English after twelve lines — there is a ‘turn’ or volta, after which there will be a change or new perspective on the preceding idea.

The metre is iambic pentameter, that is five pairs of stressed and unstressed syllables to the line. The effect is elegant and rhythmic, and conveys an impression of dignity and seriousness. Shakespeare’s sonnets follow this pattern.

Rhyme Scheme
The rhyming pattern comprises three sets of four lines, forming quatrains, followed by a closed rhyming couplet.

In sonnet 18 it forms ABAB, CDCD, EFEF, GG. This is typical of Shakespeare’s compositions. For contemporary readers today not all the rhymes are perfect because of changed pronunciation, but in Shakespeare’s time they would probably have rhymed perfectly.

See Don Paterson – Reading Shakespeare’s Sonnets, Faber & Faber, 2012
Helen Vendler The Art of Shakespeare’s Sonnets Harvard University Press
Shakespeare’s Sonnets with Three Hundred Years of Commentary, Associated University Press 2007
BBC Podcast, Melvyn Bragg, “In Our Time” Shakespeare’s Sonnets