Missions are divided into 4 main types:
1.Attack - kill zombies with sniper, do head shots and be on time
2.Waves - zombie groups are attacking, enjoy sniper shooting
3.Survive - keep yourself alive, use your sniper
4.Run - they are running, just do your job - zombie shooting
Plus you will have business missions where you can earn money or gold.
Earned money can be spent to buy better sniper guns, aids, armor and sniper bullets.
Enjoy zombie shooting and kill them before they come near!