Basically, you play as a regular soldier to begin with, with 3 default load-outs (classes) of SMG, Sniper, LMG style play. Now I don't know much about the game yet, I only gave it a go today, but at some point you become high enough to put your name forward for the role of squad leader. Then as you progress, you can put your name forward for army commander (in charge of all squad leaders).
The idea behind squads, is that each one works towards a particular objective, and this creates an obvious flaw... in the sense that if the players don't know each other, then chances are they will run off and do their own thing, Call of Duty style. However, if you play with friends, or people who are willing to listen to advice / commands, this system can work really well.
Now M.A.G works on a much larger scale than games like CoD / Battlefield, in the sense that you can play with up to 256 players on the map at once, and believe it or not, this is actually lag free, or so I am led to believe. I played a game of 32 v 32 v 32, so 96 total earlier, and it ran smoothly, but it was total chaos as nobody was listening to orders, lol.
You can create custom classes, and you can unlock new gear and weaponry by leveling up, like with CoD, but each class has a specific cost 'limit', where you can't have the best of everything in one class, as for example, you have a limit of 3000 points.
Sniper Rifle: 1200 points
Sidearm: 300 points
RPG: 800 points
Body Armour: 1000 points
= 3200 points, which is over the limit.
So you have to compromise somewhere in order to be under the limit. This helps to avoid 'overpowered' class combinations that can be found on many other games. There is also a system where each time you rank up, you gain a skill point to be spent in any area you wish to improve. The two available sniper upgrades for level 1, for example, allow for faster-reload time, or a better scope.
That's all I can remember to post right now, but I really enjoyed playing it earlier. I'm going to be playing with a full squad of people I know soon, so that will hopefully increase the enjoyment tenfold.