Bands, the simplest level of social organization, are represented by “empty” provinces. They cannot be directly interacted with (although events may allow for interaction). Bands are always simple hunter-gatherers.
Tribes are the lowest level of social organization which allow for diplomatic interaction. Tribes can be complex hunter-gatherers, pastoralists, or simple agrarians. They do not allow for most government actions, as they do not have a centralized, enforced leadership, and they can control at most one province, with no subjects. However, tribes can evolve into Simple Chiefdoms under certain circumstances.
Simple Chiefdoms have developed a centralized, enforced leadership, but that leadership's power is limited in range to one province, and they cannot have subjects. Simple Chiefdoms can be complex hunter-gatherers, pastoralists, or agrarians.
Complex Chiefdoms have developed a multi-layered hierarchy which can manage one area directly and can have single provinces (under Simple Chiefdoms) as subjects. Complex Chiefdoms can be either pastoralists or agrarians.
Empires have a much more developed hierarchy which can manage one region directly and can have areas as subjects. Empires are always agrarians, though they may have pastoralist subjects.
States have a fully developed bureaucracy which can manage an entire continent directly and can have regions as subjects. States are always agrarians, though they may have pastoralist subjects.