Watch Dogs offers several multiplayer modes but you can't access them right away. Here's what you need to do before you can start challenging other players.
The game's multiplayer is unlocked by completing the campaign mission "Hacking Contract." It won't take you too long, fortunately. Hacking Contract is the fifth mission in the campaign.
"Hacking Contract" is a tutorial for Intrusion, the one-on-one multiplayer mode. One player tries to get close enough to another to hack into his data. While the hack is underway, the other player must locate the data thief and try to neutralize him. When the hacker finishes downloading the data, he has to escape without being killed.
The victor receives Notoriety, which allows them to unlock new skills. It also bumps them up in the global leaderboards.
In "Hacking Contract," you're playing defense. You have to head to a target location to initiate the match. After reaching the area, 2XTheTap (an A.I. opponent rather than a human) will start hacking you.
You have to identify the hacker by scanning nearby pedestrians with your phone. Nearby security cameras can help you get a good vantage point on the scene. As the hack progresses, the search area will narrow to make it easier for you to find the culprit.
Personally, I just started shooting people. Eventually one of the people I killed was the hacker. I couldn't enjoy my victory for long, though, because my killing spree gained the attention of a cadre of police. It's better to be patient and methodically scan the crowd for 2XTheTap.
After you've dealt with 2XTheTap, you can open up the Online Contracts App on Aiden's phone and select Online Hacking. The game will then find a player for you to hack. This opponent is going to be a human rather than a bot so expect more of a challenge.
Your exact strategy will vary based on location. If there's an accessible roof, you could hide up there to perform your hack. If it's a crowded plaza, you might want to walk around among other pedestrians. You're not allowed to kill your target, even after the hack is complete, so make sure you've got an escape plan in mind. Win or lose, "Hacking Contract" will be completed.
Once "Hacking Contract" is done, you can access all the other multiplayer modes of Watch Dogs. Decryption is a team-based or free-for-all mode in which players try to steal data from each other and hold it as long as possible. Online Races and Free Roam are exactly what they sound like. Finally, there's a head-to-head challenge offered by the Watch Dogs mobile app. One player must race through checkpoints while a player with a mobile device tries to stop them with obstacles and police units.
Watch Dogs launched on May 27th for PS3, Xbox 360, PS4, Xbox One and PC. Find out what critics thought of the game here.