I think it's just an update they did to the base game, which was a big multiplayer only team based shooter. I think the zombies mode just replaces one of the teams with AI zombies, but I'm likely wrong on that side.
Once we perfect building Mechs, I expect there to be a factory in Mexico pumping out a brand call Mechsican. I wanna buy me a Mechsican. steamid: Rohchya