Dayz is simply put an epic zombie survival mod. Featuring over 225 square kilometers of terrain it does what L4D doesn't It really puts you into a semi realistic survival setting.
You start off with a pistol and some basic equipment, your only goal is to survive by any means necessary you require food and water, you also can get wounds which require bandages to "heal" if you get wounded your screen shakes and you require pain killers to be stable. If you get injured enough you need to scavenge blood packs for a transfusion. Which another player has to give you. Did I mention it's multiplayer only?
Well it is, so unlike stalker where you have predefined allies and enemies, everyone is an enemy or a potential ally. See someone with a better gun than yours? Kill them and take it. Just hope you don't miss otherwise you're the lootable object.
website with info.http://dayzmod.com/index.php