What are the major social structures of Sumerian society?
Sumer, the earliest civilization in Mesopotamia, was organized as a set of city-states, to a great degree mutually independent. Many of the city states were ruled by kings; the story of Gilgamesh, despite its many mythical elements, represents accurately the structure of a city led by a single monarch who had a close relationship with the patron diety of the city.
Palace and temple were the two key adminstrative entities within the Sumerian city, and closely intertwined. Property as well as power was highly centralized, as was law.
The ancient Mesopotamian law codes reveal a highly stratified society in which the gods delegate power to a king, and the king is responsible to the gods; the law codes make explicit that if the king fails to hear appeals from and protect widows, orphans, etc, that the gods will be displeased.
People were valued according to their station -- how much you were fined for injuring someone depended on whether the person was male or female, noble, free or slave.
The temple was a center not only of religious practice but of literacy education and 'scientific' medical knowledege. Diviners from the temple could be summoned to individual's houses to determine causes of diseases.
Literacy was limited to members of a scribal profession, which like other professions was primarily hereditary in the sense that sons normally followed the professions of their fathers.