The premise is simple: you'll play in teams working together towards a common goal, but also in competition too. For example, the trailer shows how a goal might challenge each member of the team to score five headshots first, or steal another member's item in secret.
The mode looks like a ton of fun, and it's a clever twist on the standard co-op/competitive multiplayer gameplay. To be fair, the last Call Of Juarez game had pretty clever multiplayer too. We're just still not sure about the modern setting.