love/ly and/ more tem/per-ate:/    

Rough winds/ do shake/ the dar/ling buds/ of May, /  

And sum/mer's lease/ hath all/ too short/ a date./    

These four lines are interesting in the sense that Shakespeare does not vary from a strict representation of iambic pentameter in each line. There are no caesuras (pauses) or feminine endings (where there is one or more unaccented syllables after the fifth stressed syllable in a line). This is unusual for Shakespeare, perhaps, and demonstrates the speaker's unquestionable devotion to his love and his insistence that her beauty exceeds what nature can offer; he has created a perfectly regular representation of it. The consistent use of iambic pentameter certainly captures the nature of this enduring beauty.

Shakespeare's "Sonnet 18" is written in iambic pentameter and is a perfect model for the Shakespearian sonnet.  Meter in poetry charts the rhythm of the poem's words and depends upon the pattern of stressed and unstressed syllables in each line. 

In "Sonnet 18," the meter is iambic pentameter; this means that each line is composed of five iambs (penta=five).  An iamb is a metrical foot.  The metrical foot is a term used to describe the rhythm of two syllables.  For example, an iamb is made up of an unstressed syllable followed by a stressed syllable (like ka-BOOM).  Shakespeare maintains perfect iambic rhythm throughout each of the lines.  Here is one of the lines broken into iambs:

Shall I | com-pare | thee to | a sum | mers day?     

Thou art | more love-| ly and | more tem-|per-ate