This article contains speculation, information which has not been confirmed by a reliable source. The game is still in development, and all information is subject to change.
Concept art of a Chosen Battleframe
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 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.
The Chosen seemingly work in tandem with Titans, large, spider-like creations, as seen in this gameplay video. How, why and what function the Titans fill in the Chosen's army is up for speculation.
Some speculate that the Chosen were once human, but are now corrupted by the Melding into loyal minions. The Affinity Manga seems to indicate this as well. This rumor is still awaiting confirmation, but it seems unlikely it will receive any.
According to an interview with Mark Kern[1], the Chosen will not be a playable race in Firefall.