When you begin the game you'll be offered one of the several tasks: Kill Zombies, find supplies, and find survivors.

If you decide to kill zombies, you'll be given a minimum number of zombies to kill before your mission is complete, and you're given exp. reward.

If you search for supplies you'll have to find and push a supply crate back to your base. You'll be rewarded with additional health packs and exp.

If you search for survivors, you'll have to find and lead someone back to your base for an exp reward.

You can choose to cancel a mission at any time, and when you finish with a mission you can either continue the same day or rest until the next.