It's criminals VS enforcers (Police, basically)
Players can make there own groups in either faction, like in the screenshots you can see the gang names "Sons of Guns" and "The Dolls", in the upper right corner. Well those gangs commit crimes and can in engage in gang wars with other criminal gangs. When events like this happen, the enforcers get a notice about it (An All Points Bulletin or APB) and respond accordingly. All this happens seamlessly of course.