Here's the homesite for the mod http://www.dayzmod.com/
It's like a FPS type game but like no other. Imagine a videogame modeled after the TV series The Walking Dead, this is what it would be.
This mod is only in Alpha stage and has a few random bugs, but even with those and not a finished mod or even in Beta yet, it's pretty freaking awesome. It's being developed by one of the ARMA series lead developers, so it is not an add on or a partial modification, it is a complete modification.
Expect to be lost at first, the learning curve is steep, but once you get the general idea of how stuff works and a general idea of navigating it becomes pretty fun. Teaming up with others makes the experiance even more fun. Expect paranoia, I'm freaking half scared to go out on my own once I get kitted back up.. LOL
The only objective of the game is to survive and see how long you can keep your character alive! When you die, you start back from scratch again.