Watch Dogs is out on pretty much every platform but the Wii U, and you can grab the mobile app on top of that today to augment the game. As a "second screen experience," you'll be able to see the map on your device, as well as the power to play a light multiplayer session.
It drops today on iOS and Android devices, and it'll let you play over your data network or wifi. Basically, you just log into the app with your appropriate account and start playing.
Watch Dogs - CTOS Mobile App FAQ [Ubiblog]