DayZ is a mod for a game called ARMA 2.  Combined operations is needed to run DayZ.  ARMA 2 is basically an army simulator   It is nothing like Call of Duty, because it is much more realistic.  DayZ turns the map into a world full of zombies.  The game is online with many random other people most servers hold tops of forty people or so.  When you first log in you spawn at random on the map with hardly any supplies and no weapon. From there you will need to sneak your way through a city to find loot without the zombies finishing you off.  The loot could be food, weapons, a map or any useful supplies, sometimes you will get really unlucky and find nothing.  Yes you will have to eat and drink to survive, maybe even receive a blood transfusion  Other players usually account for most of the player kills though, but if a player kills enough they will have a bandit skin which obviously makes them look different.   Unlike most other games when you die you lose everything and have to start over completely from scratch.  This game is quite fun but it is good to meet up with a group of friends for safer travel.