What form does "Sonnet 18" take, and how does this powerfully convey its message?

As its title indicates, the form of "Sonnet 18" is a sonnet. As with many of Shakespeare's sonnets, it is specifically an English or Shakespearean sonnet. This means it is written in iambic pentameter. Its lines consist of five iambic feet; an iamb is an unstressed followed by a stressed syllable. In terms of rhyme, it consists of fourteen lines, set up as three open quatrains followed by a couplet. The rhyme scheme is ABAB CDCD EFEF GG.
The sonnet is a form typically used for love poetry, which is true of "Sonnet 18." The Shakespearean sonnet typically has a turn after the third quatrain, with a surprise or shift in focus in the couplet. In this case, the shift is from the notion of the mortality of worldly objects and the beloved, to the permanence of the poem that immortalizes the narrator's love.