The Chosen are a newly discovered race, of what seems to be humanoids, that appeared out of the Melding in the events preceding the game's timeline. These warriors are hell-bent on the destroying the remnants of the human race and their settlements. Why and exactly where they originate from is uncertain. Since they arrived from inside the Melding, however, it seems that they are somehow connected to it.
Some speculate that the Chosen were once human, but are now corrupted by the Melding into loyal minions. This rumor is still awaiting confirmation, but it seems unlikely it will receive any.
According to an interview with Mark Kern, the Chosen will not be a playable race in Firefall.
The nature of the Melding is still clouded in secrecy. In the CG-trailer the Melding is supposedly the big, looming cloud, but the exact nature of this cloud is still unknown.
The Melding, it says on the Firefall front page, is an "inadvertently" invited "enemy". This seems to imply that the Melding is some sort of lifeform, but this has not been confirmed. In a thread concerning the name of the game, Firefall, it was explained by a Red 5-employee that Firefall was an event preceding the Melding. Thus some think of the Melding as a description of the event where the gigantic cloud arrived on earth, although the term is apparently also used to refer to the cloud itself.
It is also speculated that the Melding (the cloud/organism) has some sort of connection to the Chosen and to the bugs that now inhabit the planet, although this is pure speculation.
More information and lore has been promised by Red 5 returning from PAX. To be continued.
*Note: This information is collected by Thaewyn in Firefall Wikia.