Societies need rules to make them work. Most of our community activities are governed by a set of laws restricting or licensing our behaviour, and stepping outside those laws usually brings some consequence to bear. We conform also to a system of ettiquette, which tells us what is polite or acceptable or rude; a system of rules for certain social situations and circumstances.
Online communities are no different, they need rules to operate successfully. Often termed netiquette, these systems of rules differ from community to community, are seldom written down, but play an important role in the virtual lives of the participants.