You can see the efects and wich area of influence in the city scroll. Happiness increse public order, as same law and city garrison, also helps decreasing culture penalty, sometime gives boost on population growth, like entertainement buildings.
Unrest is a result of armies passing by in the region, or rebels bloking roads, or ports. Sometimes unrest is influenced by culture or distance from capital, as well enemy spies in the city. Other times is a certan character in the city who gives negative efects on the population and public order.