When speaking of English poetry, there are four kinds of sonnets to think about. These are the Petrarchan sonnet, the English sonnet, the Spenserian sonnet, and the Shakespearean sonnet. Sonnet 18 is of the fourth type.
Petrarch was the Italian originator of the sonnet form. As he developed the sonnet, it has 14 lines and a change of idea at the 9th line, which is called the volta, meaning turn in Italian. It opens with an octave followed by a sestet; has no couplets; and has a complex rhyme scheme, with a fixed interlacing rhyme scheme in the pattern abbaabba in the octave and one of several schemes in the sestet: cdcedc, cddcdc, and cdeced etc.
Written as a cycle, or sequence, Petrarch wrote a series of sonnets over a span of years in the same persona and tone and to the same subject. Each was built on contrasting metaphors, the first being introduced in the octave with the contrast introduced at the volta. It is written in iambic pentameter. Later sonnet forms retained the elements of love cycle, contrasts, metaphor, volta, and iambic pentameter; the octave / sestet structure was traded for quatrain / couplet structure.
English poets eventually adopted Petrarch's sonnet form, changing it in some regards to suit the English language and subject matter. The sonnets retained their character as loves poems and maintained their structure of 14 lines with contrasting metaphors. Chief sonneteers were Thomas Wyatt and the Earl of Surrey. Thus they introduced the English sonnet, the second type, which has a structure of three quatrains and an ending couplet and is also in iambic pentameter.
Edmund Spenser was the first to significantly modify the sonnet form, enough so that the third type is named after him. The Spenserian sonnet has some intriguing complexities including concatenation that chains the rhyme scheme of the three quatrains to each other: ababbcbccdcd ee. Concatenated lines carry the /b/ rhyme into the second quatrain and the /c/ rhyme into the third. Thus, three couplets are created (bb cc ee) along with greater freedom with the contrasting elements.
The fourth type is the Shakespearean sonnet, which is the earlier English sonnet of Wyatt and Surrey polished to perfection. As in Sonnet 18, the Shakespearean sonnet is iambic pentameter with 14 lines and three quatrains, with changes of idea, or volta turns, at lines 5 and 9. The rhyme is abab cdcd efef gg, with a single couplet. It relies on an opening metaphor and a contrast introduced at line 9 (optionally also line 5): "But thy eternal summer shall not fade." The resolution to the problem and contrast of the three quatrains is in the couplet:
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.