The three estates & the Great Chain of Being
The structure of medieval society from The World of Medieval Europe
The Greek Elements and 4 Humors (stop reading after the 4 Humors section!) from The Proceedings of the Friesian School, 4th series
4 Humors, Allegory, Love, Liberal Arts from Hanley's Chaucer Scriptorium
For Further Reading:
Medieval Women:
an
Interactive
Exploration
Medieval
Bestiary: Animals in the Middle Ages