Zeus has the power to create and destroy, you even get ability to possess A.I. units or shoot lightning bolts at anything you wish (however there is a timer on how often you can shoot lightning).
There is only 1 slot for Zeus and 1 for moderator (not sure what it does).
as Zeus, you are in control, while other players fight for survival, you can spawn in A.I. units to cover players or make them attack players, there are 4 sides, BluFor, OpFor, Independent and Civilian.
Zeus mode is basically live map editor, you have the same powers as in map editor, you can even spawn buildings and build entire city or destroy one. You can customize weapon crates, spawn them, create missions ..etc ..etc
As said, you have same power as in map editor, only its way easier and simplified. It's simply EPIC DLC and its FREE!!
Download : ZEUS is official DLC and it should download with latest ArmA3 patch (10.apr.2014), which is about 500mb and it will be available under "Scenarios" in single player and in multiplayer you can join any server running Zeus or just create new session and it will be listed as a mission.
If on some reason it does not show up in-game, then check out ArmA3 Workshop on steam.