it used to be an arma mod, but it's a standalone game published by bohemia now. It's currently in alpha and is quite buggy, but very fun. The mod is still around and actually has more content atm, but is less polished (that will change in the future as the standalone develops). It's really fun and the tense situations you get into with other players are very unique. I would recommend it, just don't go in expecting a finished game, it still has a lot of bugs, and most features aren't implemented.