This is a fast-paced cooperative survival map with more of an arcade feel than a gritty/realistic one. The objective is to kill as many enemies as possible to score points before you're evacuated. Kill streaks result in more points. Higher points means higher ranks, better endings, and tougher enemies!
Here's a gameplay video of a very short game where the players didn't get any time extensions. Games can go ~25 minutes in Mercenaries mode. In Survival mode, the game only ends when all players are dead for 10 consecutive seconds.