The Sonnet (specifically, the Shakespearean Sonnet) is a specific poetic structure with fourteen lines.  
Essentially, a sonnet combines three quatrains ABAB with a couplet.  The couplet usually forms a clincher of some sort.  See more

My girlfriend's eyes are nothing like the sun;
Coral is far more red than her lips' red:
If snow is white, why then her breasts have none;
If hairs are wires, black wires grow on her head.
I have seen roses streaked red and white,
But no such roses see I in her cheeks;
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
I know although I love to hear her talk 
That music has a far more pleasing sound:
I admit I never saw a goddess walk,  
when My mistress goes, she plods upon the ground:
And yet by heaven, I think my love so rare,
And those who are false cannot compare.  

                                                   William Shakespeare
(paraphrased) 

My mistress' eyes are nothing like the sun;
Coral is far more red than her lips' red:
If snow be white, why then her breasts are dun;       dun=grey
If hairs be wires, black wires grow on her head.
I have seen roses damasked, red and white,
But no such roses see I in her cheeks;
And in some perfumes is there more delight
Than in the breath that from my mistress reeks.
I love to hear her speak, yet well I know
That music hath a far more pleasing sound:
I grant I never saw a goddess go,                               grant =admit
My mistress, when she walks, treads on the ground:
And yet by heaven, I think my love as rare,
As any she belied with false compare.  

                                          William Shakespeare
(original) 

I refuse to admit there are obstacles 
To the marriage of true minds.  Love is not love
If it changes when conditions change,
Or breaks when meddlers meddle:
Oh no! It is the North Star 
That looks on storms but it never shaken; 
It is the hope of sailors near and far, 
It's value's untold, although its altitude's measured.
Love is not the toy of Time, though rosy lips and cheeks
Will come within range of his knife.
Love does not alter with hours and weeks,
But endures instead to the end of life.
If I am wrong and it can be proved
Then I never wrote, and no one ever loved.

                                          William Shakespeare
(paraphrased)

Let me not to the marriage of true minds 
Admit impediments. Love is not love 
Which alters when it alteration finds, 
Or bends with the remover to remove. 
O no, it is an ever fixed mark 
That looks on tempests and is never shaken; 
It is the star to every wand'ringbarque, 
Whose worth's unknown although his height be taken. 
Love's not time's fool, though rosy lips and cheeks 
Within his bending sickle's compass come; 
Love alters not with his brief hours and weeks, 
But bears it out even to the edge of doom. 
If this be error and upon me proved, 
I never writ, nor no man ever loved. 

                                       William Shakespeare
(Original)

When I'm unlucky and I'm despised,
All alone I weep to be so stuck. 
My prayers to heaven are useless cries. 
I look at myself, and curse my luck.   
I wish I were like one whose future was brighter. 
I wish I looked like him that has friends all `round.
I wish I had this man's skill and was a better writer.
What I should most enjoy, only brings me down.  
But, with these thoughts - myself almost despising,
I happen to think of you.  Then my mood
(Just like the lark at break of day rising
From lowly earth) will sing hymns to heaven;
For remembering your sweet love such wealth brings
That I'd hate to change my place with kings.

                                     William Shakespeare
(paraphrased)

When, in disgrace with fortune and men's eyes,
I all alone beweep my outcast state
And trouble deaf heaven with my bootless cries
And look upon myself and curse my fate,
Wishing me like to one more rich in hope,
Featured like him, like him with friends possess'd,
Desiring this man's art and that man's scope,
With what I most enjoy contented least;
Yet in these thoughts myself almost despising,
Haply I think on thee, and then my state,
Like to the lark at break of day arising
From sullen earth, sings hymns at heaven's gate;
For thy sweet love remember'd such wealth brings
That then I scorn to change my state with kings.

                           William Shakespeare
(Original)