There is a lot of news around about the upcoming 4.3 patch and everyone is talking about pretty much everything. That is why I was thinking that it would be interesting to gather some information about Deathwing.
I'm sure a lot of people do not know too much about Deathwing, especially how he became such an evil character. I'll try to keep this short because I know most of the people that are not into Lore do not like long stories...

The Past

Thousands of years ago, the titans created the five aspects. These are the five dragon protectors of Azeroth and they were created from a dragon known as Galakrond. If you want to see the remains of this ancient dragon, you can see them in Dragonblight at Galakrond's Rest.
Deathwing used to be one of the aspects, though at that time he was known as Neltharion, the Earth Warder. He was the aspect responsible for the dominion of earth on Azeroth. He shaped the old world, creating the mountains and the rivers for the mortals to live in.
Among his offspring were Onyxia and Nefarian and his consort was Sintharia, known as Lady Sinestra for most of the players.

The Old Gods drew Neltharion mad and convinced him to create a powerful artifact, the Dragon Soul, by using his own essence. Neltharion brought the Dragon Soul to the other dragon aspects and tricked them to give their essences too.
Using the tremendous power of the newly empowered artifact, Neltharion fought the Burning Legion and then demanded that all dragons should bow down to him, killing most of the ones that refused to obey.

The Dragon Soul corrupted Neltharion and began to tear his body apart. He had to cover his entire body with adamantium plates to hold himself together.
And so, the Dragon Soul artifact he created became known as the Demon Soul and Neltharion became Deathwing.

Malfurion Stormrage was the first to succeed in taking the artifact from Deathwing. After a while, the other Dragons managed to get their hands on it and to place a spell on the Demon Soul so Deathwing was unable to use it any further.

During the Second War, Deathwing led the Dragonmaw Clan to the Demon Soul and the clan used it to capture Alexstrasza and use her red dragons to fight for the Horde.
Deathwing infiltrated the Alliance as Daval Prestor. He wanted to destroy the Alliance from within. Krasus discovered the true identity of Daval Prestor and with the help of the other remaining aspects he freed Alexstrasza. Deathwing was forced into hiding, so he retreated to the elemental plane of earth, Deepholm, to recuperate.

The Present

Deathwing returned recently with the help of the Twilight's Hammer clan. They managed to improve Deathwings armor by using Elementium for the plates that cover his body.
His return is known as the Shattering, because the entire world was affected by his explosive emerge from Deepholm.
Currently he roams the world spreading death all around him, the other aspects are pretty much powerless against him.

In order to stop him a group of heroes aided by Thrall will have to find the Demon Soul and use it against Deathwing.

The Game

Deathwing was first seen during the Yogg-Saron encounter. He appears in his human form and the players can see him how he created the Dragon Soul.
Cataclysm is basically HIS expansion. Players can see him in his human form while questing in Badlands and in his dragon form in Twilight Highlands, where he fights Alexstrasza and nearly kills her. Players can also encounter Deathwing randomly in most of the areas of Azeroth and if they are not careful, they can actually die.

The 4.3 patch, named Hour of Twilight, will allow players to actually fight Deathwing in an epic encounter.
In order to defeat Deathwing, players will have to recover the Demon Soul and use it against him. Basically, you'll have to defeat him piece by piece.

 
I tried to condense as much info as I could and keep this short. I'm sure I forgot a lot of things and maybe some important facts, especially from Deathwing's past. If you see something missing please leave a comment here.

For more references check out wowpedia or wowwiki, read our Shattering review, and read up the official site.