Norms, both legal or customary/social norms, work as a coordinating mechanism for society, in that they contain prescriptions for behaviour in view of shared social space.
Norms can be thought of as external obligations, impinging and competing with other wants. Normative agents are able to recognise behavioural regularities as norms, and decide (using mental representation and reasoning) whether to comply or violate those norms -thereby reinforcing or diminishing them.
The focus on social interactions makes norms interesting to Agent-based modelling researchers. The mechanisms of social influence and social control associated with norm dynamics in populations of agents may have unintended or unexpected consequences — this could explain a lot in terms of the relationship between social change and environmental change.