Mann vs. Machine is a co-op game mode released in the update of the same name. In Mann vs. Machine, a team of six players attempts to prevent a horde of robots from carrying a bomb to their base. As well as the AI-controlled enemies and co-op gameplay, another distinguishing feature of the game mode is the ability for players to purchase upgrades for their weapons using in-game currency. All purchased upgrades will last for the duration of the game.

http://www.youtube.com/watch?v=TzxpbUeN-eg

On the highest difficulty, I don’t see how some are possible, great challenge.