The main point of the game is to hack the city's technology in order to help the main character. There isn't a linear story to follow, but the game prompts the player to perform specific actions such as stopping crimes.
The game allows to control practically anything via a handheld device. (which kind of looks like an iPhone?) The player can e.g. hack traffic lights to cause traffic to crash, hack ATMs to withdraw cash, hack people's phones to end calls etc. The possibilities are enormous.
The game was praised at E3 about it's originality, next-gen graphics and the unique gameplay. Ubisoft's executive director for EMEA territories, Alain Coore, stated that Watch Dogs will try to compete with other open-world games, such as Grand Theft Auto.
It is about a badass super hacker that has access to everyone's personal information and uses it to fuck up some serious shit.
In a nutshell possibly the best game that is going come out in 2012.