In Zombiecraft, the main objective is to live your longest. This cannot be done with out having health first! In the HUD (Heads up display), you can see the health bar, showing 10 boxes. Depending on your difficulty, when touched by a zombie you get damaged for a certain amount. Once you get hurt enoguh to the point where there are no more hearts left, you will die. This will result in the game ending and a screen appearing with the points you earned, the wave you died on, and a little statement depending on the wave you reached. buying Juggernog can give you armor, increasing your chances of living by reducing the health taken by zombies and hellsheep.
(Anyone can add a screenshot of a death screen here!)