i was confused too. this is the best explanation i could find of it.
The previously unclarified Watch Dogs Multiplayer had finally been fully explained.
According to Ubiblog, you are always Aiden Pierce, the main character, however you will appear to be a simple NPC to other human players if you jump into their game.
In Watch Dogs, when you enter someone else’s game, you will appear to the other player as “not-Aiden.” In other words, if I hack into your game, I will see you (the “hackee”) as yet another NPC-esque character on my screen. And if you hack into my game, you will continue to be Aiden on your screen, but I will see you, likewise, as a “generic” in-game character.
Not only that but it also seems there is an unrestricted choice of what you can do once you have jumped into someone elses game, examples such as ramming into another player to stop them from getting to a mission or simply being a voyeur were given.
To prevent any interruption when doing main and side missions, the multiplayer gets turned off automatically to prevent the griefers from getting in your way, however when you are not in main or side missions the multiplayer is turned back on. There is an option to remove multiplayer overall however that’s no fun.
Finally there will also be multilayer missions in which you may have to work or work for another player.
Colin also assures us there are plenty of multiplayer missions in which you’re tasked with protecting another player, so it’s not just about adversarial actions.
so it does sound like you just hack in to anyone's game and go about your business.
Wow that sounds wild. Maybe its like Borderlands 2 where you can have your game set to open, friends only, or invite only and then you can have people join with you. This is exciting!