sonnet are fairly simple, single-syllable rhymes. The first rhyme, "behold" and "the cold" in lines one and three could potentially be read as a two-syllable feminine rhyme, but most readers would find that this throws an unnatural emphasis on the word "the," and avoid such a reading. The rhymes are all full rhymes in both Elizabethan and modern English, and their simplicity adds to the force of this powerful poem.

The iambic pentameter is smooth and regular, with no variation in the ten-syllable pattern. The only slight irregularity is the scansion of line eleven:

As the deathbed whereon it must expire...

If one scans this as a perfect iambic line, the word "deathbed" sounds unnatural, with the emphasis on the second syllable instead of the first. Shakespeare's intention, therefore, seems to be to disrupt the otherwise perfect iambic rhythm with the word "death." This means that the word "deathbed" should be read as a spondee, with two long syllables giving a marked emphasis to an important word.

To begin, sonnets contain fourteen lines; the English, Elizabethan, or Shakespearean sonnet is marked by three quatrains (four line groupings) and a rhymed couplet. The rhyme scheme is ABAB CDCD EFEF GG, meaning that the last words in lines 1 and 3, 2 and 4, 5 and 7, 6 and 8, 7 and 9 and 11, 10 and 12, and 13 and 14 rhyme. 

The meter of this sonnet is iambic pentameter. What this means is that each line of the sonnet is comprised of ten syllables, and those ten syllables are divided into five pairs.  The pairs are called iambs. Each of these iambs follows a regular pattern of a stressed (long) syllable followed by an unstressed (short) syllable when the words are pronounced. This matters because sonnets are meant to be heard, not just read. The word sonnet, in fact, derives from Italian and means "little song." 

As others have stated, the meter this sonnet employs—like all Shakespearean sonnets—is called iambic pentameter. The word iambic refers to to the type of foot used: an iamb. An iamb is made up of two syllables: an unstressed (also called unaccented) syllable followed by a stressed (also called accented) syllable.

The following are some examples of words that are, by themselves, iambs: behold, depict, destroy, employ. Their first syllables are unstressed, and their second syllables are stressed (and bold, so you can see where the accented syllable begins).

The word pentameter refers to how many feet, or iambs, are in each line.  Penta- means five; therefore, each line in the poem has five iambs (or ten syllables).  

That time | of year | thou mayst | in me | be hold
When yel | low leaves, | or none, | or few, | do hang
Up on | those boughs | which shake | a gainst | the cold,
Bare ru | in'd choirs, | where late | the sweet | birds sang

In order to demonstrate the iambic pentameter, I have divided each of the lines above into its five feet, each foot containing two syllables, one unaccented followed by one accented (in bold font).  

"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. 

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.

If you have read any of Shakespeare's other sonnets—or indeed, any sonnet in the traditional form—you may recognize this meter. It is iambic pentameter, a meter in which each line contains five iambic feet, or iambs.

An iamb is an unstressed beat followed by a stressed beat (or emphasized syllable). So, iambic pentameter does not mean that there are only five syllables in a line, but that there are five points of emphasis. Let's look at a line from this poem poem:

That TIME of YEAR thou MAYST in ME beHOLD.

The capitalized syllables represent the places where we'd give stress to the sentence if we were reading it aloud.

Now, the meter in this poem is perfect—all the lines conform exactly to the iambic pentameter guideline. However, sometimes we will find a poem written in iambic pentameter where one or two of the lines doesn't quite fit—maybe it has four iambs, or six. In such a case, we would still describe the poem as being written largely in iambic pentameter.

I am going to assume that the question is asking about "Sonnet 73" by William Shakespeare.  Being a Shakespearean sonnet, this poem's rhythm and meter is iambic pentameter.  An iambic foot is composed of an unstressed syllable followed be a stressed syllable.  Shakespeare will use this rhythmic foot five times per line, and that gives each line 10 total syllables or five iambic feet.  That is why it is called iambic pentameter.  Let us look at the first line of the poem to illustrate this point.  I will use bold to illustrate the stressed syllables.  

That time / of year / thou mayst / in me / behold

"Sonnet 73" will use this particular rhythm and meter through most of the poem. However, line 13 messes with the iambic foot at the beginning of the line. The first syllable is stressed, and the following syllable is unstressed.  This is called a trochee, and it is the opposite of the iambic rhythm.  

Thisthou / perceiv'st/ which makes / thy love / more strong.

This sonnet is written in iambic pentameter with the following rhyme scheme: the first twelve lines of the poem consist of three quatrains (verses of four lines) with an ABAB rhyme scheme. For example, in the first quatrain, the words at the end of the first and third lines rhyme: "behold" and "cold," as do the end words of the second and fourth lines: "hang" and sang." The last two line of the poem are a couplet, with "strong" and "long" rhyming.

The regularity of the da-Da da-Da beat and the end rhymes creates a pleasing sense of rhythm that structures the poem. The first quatrain describes a late fall day at twilight, where all is stark and cold. In the second two quatrains, the speaker refers to his own old age in comparing himself to the ruined, dying landscape. In the final couplet, the "turn" comes: the speaker leaves his beloved with the thought that their love is all the more strong from knowing their time is short.