Infinity Ward have released a new update for Call of Duty: Ghosts, and amongst the expected changes to weapon balance and gameplay fixes, there's a new 'Chaos' mode for the game's co-op Extinction levels.
Chaos Mode was first added back in Modern Warfare 3, setting players up against never-ending hordes of enemy soldiers. You could chain kills together to unlock perks and power-ups. Added alongside the new Chaos option in Ghosts are a variety of different improvements, the full list of which you can find here.