The sonnet is perhaps the most well known poetic form. The oldest form is called the Italian or Petrachan sonnet after the 14th century poet Francesco Petrarca (Petrach), the first major poet to use it. 

A sonnet contains 14 lines, but everything with 14 lines is not a sonnet. The rhyme scheme is critical in all forms, and how the poem splits largely defines the different types. A large number of sonnets are written in iambic pentamer. This indicates the beat, or foot (an iamb is a short-long foot) and pentameter means 5 feet per bar. So I've taken the second line of Elizabeth Barrett Browning's Sonnet 43 from Sonnets From The Portuguese to illustrate (the beat, highlighted, is stronger than the first line):

I love thee to the depth and breadth and height. 

However, the rhythm is less important - even Shakespeare has one in tetrameter (4 feet per bar)

In the Italian form, the poem breaks down into two parts. The first is a set of 8 lines (an octet), the rhyme scheme was originally A-B-A-B A-B-A-B, but later evolved to A-B-B-A A-B-B-A. A minor break falls after line 4. The next 6 lines (the sestet), usually rhyming C-D-E C-D-E, again breaking naturally into two halves, in this case of three lines each.

It is the tension between the two unbalanced halves that distinguishes this type of sonnet - the first 8 lines set the stage, in two 4 line steps. Then the sestet resolves it, in two halves. Proposition: Solution. Thesis: Proof. 

All this is ideal. It rarely occurs in English. Most commonly, there is variation in the rhyme scheme of the sestet. Wordsworth, for example, wrote many sonnets, but had varying rhyme schemes in the sestet, for example C-D-C-D-C-D. The problem this creates is that the sestet then logically falls into 3 sets of 2 lines instead, which tends to break the rhythm, and can make it seem a little bit more like an English sonnet (of which Shakespeare's 150 are the most famous. I have Sonnet 130 as the theme poem above the list of poems in the section "Of Love..."

My most successful attempt at this form was a set of 9 pieces called Symphony For the Sun, written probably in 1976. The first 3 used the standard Italian rhyme scheme, the second used Wordsworth's sestet variation, while the final 3 reverts to the pure form. Coming back to them, I tried a bit harder to get them into italian form, with breakss consistent and rhyme correct. There are notes on the whole series and the individuals attached to the poems themselves.

Browning's Sonnet 43 is one of the most well know first lines in English literature, rivalling Shakespeare, so I quote it in full below. It appears to fit as a Wordsworth variation, but the break does not come at line 8. Indeed it does not even come after line 12 (standard Shakespearean form) but half way through line 13, making it epigrammatical in the extreme. Analysis of the form is useful, but read any poem aloud and don't get hung up about the rules. Sometimes the poem is just "right" whether it fits the form or not.

Sonnet 43:
How do I love thee? Let me count the ways.
I love thee to the depth and breadth and height
My soul can reach, when feeling out of sight
For the ends of Being and Ideal Grace.
I love thee to the level of everyday's
Most quiet need, by sun and candlelight.
I love thee freely, as men strive for Right;
I love thee purely, as they turn from Praise;
I love thee with the passion put to use
In my old griefs, and with my childhood's faith;
I love thee with a love I seemed to lose
With my lost saints,—I love thee with the breath,
Smiles, tears, of all my life!—and, if God choose,
I shall but love thee better after death.

Variations appear to be far easier to find in English than the original Italian, mostcommonly the Wordworth rhyme scheme. One that is very much in the feel of the original style is The Grave Of Keats by Oscar Wilde. The sestet rhymes C-D-E E-D-C, which adds variety but maintains the original balance of 4/4 then 3/3

The Grave Of Keats:

Rid of the world’s injustice, and his pain,
He rests at last beneath God’s veil of blue:
Taken from life when life and love were new
The youngest of the martyrs here is lain,
Fair as Sebastian, and as early slain.
No cypress shades his grave, no funeral yew,
But gentle violets weeping with the dew
Weave on his bones an ever-blossoming chain.

O proudest heart that broke for misery!
O sweetest lips since those of Mitylene!
O poet-painter of our English Land!
Thy name was writ in water—it shall stand:
And tears like mine will keep thy memory green,
As Isabella did her Basil-tree.