In addition, the quatrains of the Shakespearean sonnet each express a single, differing thought.  For instance, in the first quatrain, Shakespeare places the young man beyond comparison:  "Shall I compare thee..../Thou art more lovely and more temperate:" 

Then, in the second quatrain, the poet draws a series of implicit analogies regarding the young man's temperament, likening the heat of the sun to the man's self-will, and "nature's changing course" to the coolness of the young man toward the narrator: "By chance, or nature's changing course untrimmed:"

Again, in the third quatrain the poet places the young man outside of a natural context as he declares, "But thy eternal summer shall not fade...." because of the eternal lines of poetry.

Finally, the couplet makes the suggestion that the "eternal lines" will preserve the youth, although the temperament metioned in the second quatrain ("too hot the eye" )challenges the idea that the young man will be satisfied with the reliance of the poet's "lines." 

A Shakespearean sonnet is made up of three quatrains (a quatrain is a group of 4 lines of poetry) followed by a couplet (2 lines of poetry that rhyme).  The rhyme scheme of the sonnet is ABAB CDCD EFEF GG, and the rhythm is in iambic pentameter.  Iambic pentameter means that each line of the poem consists of 5 "iambs" -- 10 syllables total, with each set of two syllables carrying the pattern of an unstressed syllable followed by a stressed syllable.  The word "compare" is an example of an iamb - the syllable "com" is not stressed when the word is spoken, by the syllable "pare" is.

Traditionally, all sonnets, including Shakespeare's, are love poems, and this one is no exception.  The poet is expressing admiration for his subject by exclaiming how much the subject surpasses even a summer's day in loveliness and beauty. 

Here is the breakdown of the quatrains:

Shall I compare thee to a summer's day? (A)
Thou art more lovely and more temperate: (B)
Rough winds do shake the darling buds of May, (A) 
And summer's lease hath all too short a date: (B)

Sometime too hot the eye of heaven shines, (C)
And often is his gold complexion dimm'd; (D)
And every fair from fair sometime declines, (C)
By chance or nature's changing course untrimm'd; (D)

But thy eternal summer shall not fade (E)
Nor lose possession of that fair thou owest; (F) 
Nor shall Death brag thou wander'st in his shade, (E) 
When in eternal lines to time thou growest: (F)

So long as men can breathe or eyes can see, (G)
So long lives this, and this gives life to thee. (G)