The setup: Your typical zombie survival scenario EXCEPT for the fact that the focus is not on actually killing zombies. Much like The Walking Dead comics, this mod is primarily about human interaction/emotion.
When you start off you have a pistol with a couple mags, some water and a can of beans. You have no map/compass and have no idea where you are. Naturally, food, weapons and other useful items are scarce, so scavenging/hunting/trading is the only way to survive.
The stakes: You can only play one character at a time and death for each character you play is permanent. If you've survived for weeks out in the wilderness (all of your character's items/info are saved to a master server), only to be shot in the head by a marauding bandit player, that character is gone forever. When you respawn, you will start a new character.
Player interaction: Most of your time in DayZ will be spent building relationships and deciding who to trust. Some people want to live in a friendly community, some will do whatever it takes to survive (even if that means killing you for a can of beans because they were on the verge of starvation) and some just want to watch the world burn. I've seen small groups of friendly survivors get robbed by masked bandits and used as zombie bait/distraction. I've seen people get murdered for a can of Coke. I've seen people trust others, only to be shot in the back and stripped bare as they lay bleeding on the ground. This is the reality that DayZ presents.
The mod is still in Alpha so there are still some bugs to be sorted out, but if you like the idea of a true survival sandbox, I can't recommend this mod enough.
By the way this all takes place on the entirety of Chernarus' 225 sq km.