William Shakespeare's Sonnet 18 "Shall I compare thee to a summer's day?" is a completely regular English or Shakespearean sonnet.
In terms of meter, Sonnet 18 is written in iambic pentameter, meaning that each line consist of five feet each containing an unstressed followed by a stressed syllable with a limited number of variations.
An English sonnet's rhyme scheme consists of fourteen lines in the form of three open quatrains followed by a couplet. The three quatrains are rhymed ABAB CDCC EFEF. Shakespeare uniformly rhymes accented syllables using full rhymes, although the odd-numbered lines of the first quatrain use a long vowel rather than a vowel plus a consonant for the rhyme sound ("day-May"). The final couplet is described as "GG" when one discusses the rhyme scheme because it contains the seventh unique rhyme sound of the poem. The rhyme words are "see-thee" which like the first rhyme words of the poem are long accented vowels, which are considered legitimate full rhymes in English poetry.