This poem is all about Shakespeare writing about his beloved. There is controversy as to whether Shakespeare is addressing this poem to a man or woman - male romances were quite common during the Elizabethan Era.
This sonnet starts off with what I would think is a rhetorical question: "Shall I compare thee to a summer's day?" It is also a simile, because if you think, you are comparing the beauty with the summer's day, literally saying: "Are you as beautiful as the summer's day".
The poet then goes on to say that his beloved is 'more lovely and more temperate (moderate and self-restrained)' than a summer's day. Negative things are said about summer, to contrast it with his beloved. All this relates to the beauty of his beloved male friend. For instance, "Rough winds do shake the darling buds of May" means winds destroy the beautiful buds of May; "And Summer’s lease hath all too short a date" means summer is too short; "Sometimes too hot the eye of heaven shines" means the sun is too hot at times; "And often is his gold complexion dimmed" means and often goes behind the clouds - like you are hiding your beauty. In the Elizabethan Era, fair skin was admired. This line talks about how you could lose this attribute due to ageing. "And every fair from fair sometime declines" means everything beautiful will eventually lose its beauty and "By chance, or nature’s changing course untrimm’d" means by old age or nature's planned course.
Then on the other hand, the poet says that his beloved is not like the extreme summer days, that his youth will not fade, he will not lose the beauty he currently possesses and beauty will not die, but live forever (and that he will be immortal), at least in this poem. As long as people exist on earth, this poem will 'live' and his beloved will too.
As stated before, William Shakespeare's Sonnet No.18 is by far one of the most famous sonnets of his, if not, even the best and most well-known. It is a dilemma to whether Shakespeare had written this to a man or a women. In my perspective, the language in this sonnet sounds especially feminine, the word 'lovely' wouldn't really be used if you were writing a poem to a male. But there is evidence that this sonnet could've been addressed to a man. For instance, in this poem: "And often is his gold complexion dimmed". Another piece of evidence is from the facsimile of the original printing of this Sonnet, http://en.wikipedia.org/wiki/Sonnet_18: "So long as men can breathe or eyes can see".
Shakespeare's Sonnets have 14 lines. There are 3 quatrains (4 lines for each stanza) and 1 couplet (2 lines for each stanza)....