What is the style, technique, and language of "Sonnet 18"?

Shakespeare's "Sonnet 18" is written in the style of a Petrarchan sonnet. In a Petrarchan sonnet, the first eight lines pose a problem or a question, and the last six lines offer a solution or answer. In "Sonnet 18," the problem is that the speaker can't find the right words to capture and preserve his lover's beauty. His lover's beauty seems incomparable and elusive. In the final six lines, however, the speaker suggests that the best way to capture and preserve his lover's beauty is by writing this sonnet for them. In this way, their beauty shall endure—beyond summer and beyond death.
In terms of the language of the poem, the speaker relies heavily on metaphor. He describes, for example, his lover's beauty as "thy eternal Summer [which] shall not fade," and he personifies death when he says that his lover shall escape "his shade." The metaphor describing his lover's beauty as summer implies that it is warm, vital, and bright. Personifying death implies that death is an adversary to be defeated.
The poem also concludes, as is conventional of a Shakespearean sonnet, with a rhyming couplet. The rhyming couplet suggests a finality and a sense of closure. In this particular poem, the speaker has reached a sense of closure because he is now satisfied that, through this sonnet, he has found a way to preserve his lover's beauty and thus keep it from the cold "shade" of death.

Style refers to the way the writer uses languages such as word choice, syntax, and figurative language. In this sonnet, Shakespeare employs a standard level of diction (which is higher than conversational), and we see this through word choices like temperate, lease, and complexion. In addition, the poem sounds quite polished, adding to one's impression of the standard diction. He does take some poetic license to alter the typical syntax of certain sentences, such as,

Sometime too hot the eye of heaven shines,And often is his gold complexion dimm'd[.] (lines 5-6)

A more typical sentence structure might read, Sometimes the eye of heaven shines too hot, and his gold complexion is often dimmed. However, in order to maintain the rhyme scheme and meter, Shakespeare deviates from what is typical (which he frequently does in both his sonnets and dramas).
Shakespeare also employs a metaphor, calling the sun the "eye of heaven," and personifying it by calling it "him" and referring to his "complexion," as though the sun has a face. Another metaphor compares the speaker's beauty, which will never fade because it is immortalized in this poem, to an "eternal summer."

William Shakespeare's "Sonnet 18" is an English or Elizabethan sonnet; as such, it is written in iambic pentameter and contains fourteen lines comprised of three quatrains with a rhymed couplet at the end. The rhyme scheme is abab cdcd efef gg.
Techniques in the poem include:
A rhetorical question - "Shall I compare thee to a summer's day?" 
Metaphors - In comparing the poem's subject, thought to be a youth, to a summer's day, the narrator finds him "more lovely and temperate"; he also likens his youth to an "eternal summer" and says that he will achieve immortality through his poem's "eternal lines."
Personification - The sun is described in human terms. It has an "eye" and a "gold complexion."
Shakespeare's language is largely figurative in the sonnet; moreover, it is Early Modern English, which succeeded Middle English in the fifteenth century.