ctOS is a Service that controls Chicago in the Game Watch_Dogs.
ctOS is weakness of the city of Chicago, it can be hacked through various viruses, in the factories. ctOS has secret document files, communication systems from the goverment. ctOS is tied to a satelite over Chicago, it has valuable informations about the city itself, which Aiden Pearce can reveal through his phone, either by hacking: ctOS Communication Systems,ctOS Towers.
ctOS is maybe the biggest company in Chicago, Who controls the ctOS? controling it would be a way bigger problem, as system isn't very easy to hack, for normal hackers, professionals like Aiden Pearce could do it easy. If system is in wrong hands, whoever controls it, could controle the whole city of Chicago.