Player-driven story
Role-playing and resource management elements. Command your troops through a branching campaign full of twists and surprises.

Extensive multiplayer
Play the entire solo campaign cooperatively or dive into huge PvP battles for control of the whole map.

Mission Editor
The Intuituive, powerful mission editor lets you design your own missions in minutes or create intricate and unique campaigns. Infinitely extend the game!

Authentic weapon simulation
Bullet ballistics and deflection, material penetration, real-world magazine capacity, tracers, ammunition types and stopping power.

Unique AI
No scripts, no pre defined pathways. AI adapt and use the open environment on the fly. AI share information, flanking and taking cover to surprise and outsmart players.

Dynamic conversation system.
ArmA 2 integrates preset conversations with dynamic queries about the game environment. Talk to soldiers in the field and civilians caught up in the fighting. Soldiers shout and use hand gestures based on real world military doctrine.

http://www.arma2.com/overview_en.html