I bought this game on Steam for a ridiculously low price but didn't know much about the series at the time. After installing this game I tried Arma2 and was absolutely blown away by it. The copy I had bought came with some DLC namely Operation Arrowhead, British Armed Forces and Private Military Company but even if you don't have those (which I would thoroughly recommend) you will still find that this is the best War game you have ever played.
The concept of this game is it is a Military Simulator meaning unlik in Call Of Duty you will not engage enemies at ridiculously close distances instead you may find yourself in a firefight 200m-400m away from your enemy. This game also includes a realistic ballistics engine so you may find one shot will not do it as bullet drop, altitude, breathing and wind all affect where you bullet ends up downrange.
There are a huge range of vehicles (tanks, motorbikes, scud launchers, HMMWV, hatchabcks etc.) and weapons (from Russia, US, UK and other countries) and Aircraft (Helicopters, Planes, Jets) which can all be controlled by you with realistic flight physics. The vehicles and plane may be neccessary as again unlike call of duty the maps are not a few hundred metres long but sometimes 15km2 in distance and attempting to walk from one end of the map to the other will take you as long as it would in real life.
This game also combines other aspects of the battle field such as strategy and leadership. As a commander you can control every team and plan out the battle or if you would prefer you can fight as a team leader in which you will need to control the formation, targeting, engagement etc. of your fireteam which in the heat of battle can be a challenge.
Overall what you get from this game is a feeling of realism (as much as possible for a game) which completely engrosses you into the battles. However for those who feel real is not real enough you can like me download and install the Advanced Combat Environment Realism Mod to take it to another level entirely.