Preservation is the fundamental opposite of Ruin. He made a deal with Ruin to create humans, under the agreement that one day Ruin could destroy everything. He later betrayed Ruin to stop the destruction by giving most of his consciousness to imprison him and take away his body, which was discovered to be the Atium itself. This reduced him to the wisp of himself which is known in the books as the mist spirit. It is revealed that the mists are Preservation's body and power which is why they swirl around someone using Allomancy.
This article contains plot details about Book 3 and will spoil part of the ending.
Vin absorbs the mists on a few occasions. During the fight with the Lord Ruler, Vin absorbs mist allowing her Allomantic abilities to be vastly more powerful. Vin also uses the power of Preservation's body (by absorbing mist) when fighting multiple Inquisitors to power her Allomantic abilities.
Preservation had guided Vin and Elend to the Well of Ascension and mortally wounds Elend in hopes of forcing Vin to use the power instead of giving it up and releasing Ruin from his prison. Preservation appears a final time to Elend, where he attempts to convey some information. After the meeting, Preservation as a consciousness dies having used up all its power. It's mentioned that though unseen by Elend, a dark haired man lay dead in the mists afterwards.
Vin briefly becomes Preservation at the end of the third book by absorbing all the mists. She attempts to kill Ruin with this power, but only succeeds in killing herself, along with the consciousness of Ruin. Sazed ends up taking up both powers in order to save the dying world and becomes a force of balance known later as Harmony.