The White Lantern Corps. appeared near the end of the Blackest Night, after Thaal Sinestro, who had absorbed the Entity of Life, had the Entity removed from him. No sooner had this happened, then Hal Jordan absorbed it himself, and spread its power over a number of earth's super heroes, some of them Black Lanterns, thus resurrecting them (This included Kid Flash, Superman, Superboy, Green Arrow, Donna Troy, Ice, and Animal Man, as Barry Allen, Hal Jordan, and Wonder Woman, had all become other Lanterns before hand). With their combined powers, they defeated the Herald of Death, Black Hand, thus allowing the Indigo Tribe to take custody of him, and defeated Nekron, Lord of the Unliving. The White Lantern Corps disbanded soon after this, as the Life Entity vanished.

White rings have all the powers of a Green Lantern ring, except they are powered not by willpower, but by life itself. A white lantern wielder would be akin to a god in terms of power level. They also have reality/time/space warping abilities, and power over life itself.

(Could this hint at a Blackest Night counterpart skin coming soon? *gasp* Stay tuned)