This was probably the first detail we actually had about the game. It's literally the entire concept of the game that it's a shared-world shooter with seamless matchmaking for real time events that occur. So yeah, it is an always online game. You can go through the game without "teaming up" with anyone else, but there will be other people around in certain areas.
Not offline, but there is co-op. It's not Goldeneye or anything,
Yeah, new ideas are stupid! /sarcasm
You should read some about the game, it actually sounds quite interesting.