First and foremost, Watch Dogs offers a full single-player experience that you can play offline. That said, I have no intentions of playing offline as the connected experience sounds very promising. Did you know we've actually seen the multiplayer in action twice now?
At E3 you saw a character called Bixxel_44 (at the 13:54 mark above) hack into Aiden Pierce's phone, the main protagonist of Watch Dogs. The video then ends with several player names appear on a overview map of Chicago such as Valkyrie, SiREN, Fidget, Pixxel, and more. We saw this connected multiplayer again during the PlayStation 4 reveal when a security camera with the name PixxelFD over it was tracking Aiden.
Other players can seamlessly join into your game and either help or hinder your progress. There's of course an incentive to do this, as you'll earn experience points and other types of rewards that can go towards your main game.
Ubisoft didn't want to share more on how this all works, but I expect there's always going to be this level of tension at play here. You'll never know for sure whether the people in your game want to help you, or betray you at the first big cash reward.
This connected experience also extends to the companion app Ubisoft is developing for mobile devices. This app has been designed right alongside the core game since inception, and it's something I got to mess with back at last E3.
You're presented with an overhead map of Chicago, and anything Aiden can do in terms of hacking you can do here. Plus there are some things you can do with the app that even Aiden can't do. One basic example of how this could work is the person on the mobile app flipping traffic lights from green to red in order to help the main player escape from pursing cops.
There's also going to be a pure multiplayer mode of some sort, but Ubisoft wasn't ready to spill details just yet. I do know that the multiplayer puts you in the same city as you get in single-player, and they're aiming for it to be an innovative experience as they don't intend to just replicate other multiplayer modes out there.