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. 

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

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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'ring barque,
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