Sonnet 73 is a typical Shakespearean sonnet. Therefore, it is written in iambic pentameter with a rhyme scheme of abab, cdcd, efef, gg. There are three quatrains that develop the theme, or action, with a concluding rhyming couplet.
In Shakespeare's Sonnet 73, in the first quatrain, the speaker acknowledges to his lover that he is growing older:
That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang....
However, this aging and possibility of dying--"Death's second self that seals up all the rest"--should "ignite the fire" in the lover into embracing him and enjoying him more fully and urgently.
Sonnet 73 is from a subgroup that includes sonnets 18-77. While the latter ones relate in theme to time, as does Sonnet 73, the earlier ones relate to the complications that develop with rival lovers. Each one of Shakespeare's sonnets holds its own beauty, nevertheless.
"Sonnet 73," as a previous answer indicates, follows the conventional Shakespearean (English) sonnet in rhyme scheme and meter. It consists of three quatrains (lines of 4) with the scheme abab cdcd efef, followed by a rhyming couplet at the end, gg. The rhyming couplet generally serves to summarize the theme or message of the poem, or, if a conflict is introduced, resolve the conflict or envision its implications.
The meter is iambic pentameter. An iamb is one foot (two syllables) whose stress pattern is an unstressed syllable followed by a stressed syllable. There are 5 iambs in a line ("penta"meter). This results in lines that are, for the most part, ten syllables long. Such a regular, rhythmic pattern would have made the sonnet easier to memorize and recite (just as the meter in Shakespeare's plays helped actors remember their lines); moreover, the rhythm creates a pleasing listening experience.