Ruin is a Shard of Adonalsium. Its original Shardholder was Ati, but is currently held by Sazed.
Intent
Ruin seeks change. It is not inherently evil, because without death there is no change, but his destructive actions are often contrary to life existing peacefully.
Associated magic
Ruin fuels Hemalurgy, one of the three Metalic Arts on Scadrial. The solid form of Ruins's power, a metal known as atium, allows the user to see a few seconds into the future.
Individuals who are pierced by Hemalurgic spikes can hear Ruin's voice in their head. Upon his release, he was able to fully take control of such people.
Lore
The deal and betrayal
Ruin and Preservation, the two Shards on Scadrial, struck a deal to create sentient life on the planet. Preservation would have a sliver more of itself in humans, but Ruin would be allowed to destroy the world.
Preservation broke the terms of the agreement by sacrificing his mind to create a prison for Ruin's mind, preventing him from destroying the world. Furthermore, in an attempt to balance the power (since some of Preservation was now stored in humanity), he sequestered away a portion of Ruin's power in the metal atium.
Ruin encouraged the mists, left behind by Preservation, to choke off sunlight to the planet. This is what became known as the Deepness.
Touch on the world
Ruin's entrapped mind could still influence the world. In particular, he changed the prophecies left behind by Preservation in an attempt to get the Hero of Ages to release him from his prison. This led to Rashek becoming the Lord Ruler.
Ruin was released from his prison accidentally by Vin 1024 years later (due to his manipulating the prophecies), and he immediately went on a rampage to destroy Scadrial. Central to this plan was his need to overcome Preservation, which meant that he needed to find the stash of atium left behind by the Lord Ruler.
The death of Ati
Ruin's rampage was ended when he had Marsh kill Elend. Vin (holding the shard Preservation) realized she had nothing more to live for, and killed herself and took Ati (Ruin's Shardholder) with her.
The end of Ruin was Preservation's plan all along. He needed someone new, and therefore not molded to Preservation's intent, to make the sacrifice needed to destroy Ruin. Leras spent too much time holding the Preservation Shard, and was thus unable to act against its intent.
Sazed picked up both Preservation and Ruin following Vin's sacrifice, and is the current holder of both Shards.