It's pretty simple, Kill or be killed. Collect as many weapons and powerups along the way and always, always keep that multiplier up! You earn more weapons, or upgrades for your current weapons by increasing your multiplier. Every kill will increase it by 1. The multiplier will go back down over time, and this speeds up depending on how high your multiplier is. You can also play with a friend in Cooperative (fight with your friend against the zombies) or Death Match (make your friend the enemy) mode.