Shakespeare's sonnets may at first glance seem simply like poems, and for some, difficult ones at that. The process of understanding a sonnet involves careful reading. The intricacies of the sonnet become apparent when studying the structure and devices the author uses to construct the poem. Literary devices are often used with sonnets, and Shakespeare often uses what is known as a conceit, which is similar to an extended metaphor, but more elaborate.
Wikipedia discusses the concept of a conceit:
In literature, a conceit is an extended metaphor with a complex logic that governs a poetic passage or entire poem. By...manipulating images and ideas in surprising ways, a conceit invites the reader into a more sophisticated understanding of an object of comparison.
Dictionary.com explains a conceit as:
an elaborate, fanciful metaphor, esp. of a strained or far-fetched nature...the use of such metaphors as a literary characteristic, esp.in poetry
The contribution a conceit makes is to provide a clearer image of Shakespeare's (or the author's) subject and what he compares her/him/it to.
The structure of the poem provides some guidelines. In Shakespearean sonnets, Shakespeare uses quatrains (four-line stanzas) and a rhyming couplet (a pair of lines that rhyme) to organize his thoughts.
Consider Sonnet 15 by William Shakespeare:
The conceit of the first quatrain (first four lines) provides the image of things that grow, and the momentary beauty (which he will soon argue, fades).
When I consider every thing that grows
Holds in perfection but a little moment,
That this huge stage presenteth nought but shows
Whereon the stars in secret influence comment;
The conceit of "growing" (remember, like an extended metaphor) continues into the second quatrain. The poet then compares man something growing:
When I perceive that men as plants increase,
Cheered and cheque'd even by the self-same sky,
Vaunt in their youthful sap, at height decrease,
And wear their brave state out of memory;
The "shift" or pivotal point of this poem is identified with the word "then," (other times it could be "however," or "therefore," etc.). The poet turns his attention to the object of his sonnet, that which the growing is compared to:
Then the conceit of this inconstant stay
Sets you most rich in youth before my sight,
Where wasteful Time debateth with Decay,
To change your day of youth to sullied night;
The author expresses himself directly to "you," the object of his comparison, discussion how [she] is rich and young, but will, with time, decay (grow older) so that she will approach a time when she is not as beautiful as the young plant.
However, the couplet draws the comparison to a close with hope and love:
And all in war with Time for love of you,
As he takes from you, I engraft you new.
The author will not allow Time to rob her of that which is lovely, but his love will see her in new light, still as beautiful as before; love will engraft her so that she is new and lovely again.