Attacking
Attacking is just as simple. You click on a Zombie to shoot at him. The distance to the Zombie and your weapon take are taken into account as to whether you will kill the Zombie or not. When fired, three of the four weapons allow the Zombies to move.
Zombies
Zombies are here to kill you. It's what they do. They move every time you do, as well as most of the time you shoot. If they see you, they will come after you. They aren't smart though, so try moving around corners and use doors to your advantage.
Characters
This is you. Awesome.
This is a Zombie... they try to kill you.
This is a Gaurd, he signifies that the area is safe.
This is a Priest, maybe he will help you.
This is the researcher, he could help you too.
Weapons
A pistol - moderate accuracy at short and long range
A shotgun - very accurate at short range, not so at long
An AutoRifle - accurate at both long and short range, rapid fire
A Laser - this experimental weapon is always accurate
Background
These are doors. They help keep the Zombies at bay, however, the Zombies can destroy them
These are walls... you can't walk through them...
These are strairs. Use them to go between floors
This is a vent... not very useful, but add to your understanding