Bungie has previously said that Destiny will be a living world, meaning that events will be able to occur across the game that aren't necessarily precipitated by the player or even anticipated by the developers. It incorporates MMO elements, but Bungie has strenuously resisted any attempt to label it as such, calling it a "shared world" shooter instead. Players will be tasked with defending the last city left on Earth after the aforementioned apocalypse destroyed everything else.
Thus far, moving the beta to PS4 and PS3 first is consistent with Bungie's PlayStation-centric approach; the company has previously announced that the two Sony consoles would also receive exclusive content options compared to the Xbox 360 and Xbox One. A firm release date has yet to be announced, but the target is Q1 2014. No PC version is planned.