Short answers, pulled from the story.
Sonnet 18 asks whether the speaker should compare a person known as the Fair Youth to a summer's day, then argues that the Fair Youth surpasses summer because summer is temporary and subject to change. The poem promises the Fair Youth will live forever in its lines, though it contains almost no description of the young man himself, preserving instead vivid descriptions of a summer's day.
Sonnet 18 was first published in 1609 as part of a collection of 154 sonnets by William Shakespeare. It appears in the Fair Youth sequence, which comprises sonnets 1 through 126 in the numbering that comes from that first edition.
Sonnet 18 is a Shakespearean sonnet with 14 lines of iambic pentameter, divided into three quatrains followed by a closing couplet. Its rhyme scheme is ABAB CDCD EFEF GG. The poem also contains a volta, or shift in subject, beginning with the third quatrain.
The Fair Youth is the unnamed subject addressed in Sonnet 18 and throughout sonnets 1 through 126 in Shakespeare's sequence. The poem does not identify the young man by name; scholars have debated his identity without reaching a settled conclusion from the text itself.
"Untrimmed" in line eight of Sonnet 18 carries two meanings: the loss of decoration and fanciness over time, and the image of a ship with sails not adjusted to shifting winds. The second reading, combined with the phrase "nature's changing course", creates an oxymoron suggesting that the only constant in nature is change itself.
Musicians who have recorded Sonnet 18 include Bryan Ferry, who set it for the 1997 album Diana, Princess of Wales: Tribute, and Paul Kelly, who recorded it for his 2016 album Seven Sonnets & a Song. David Gilmour and Chuck Liddell have also recorded versions.