Barons: Mostly bankers and corporate executives. Anyone who lives at the undue expense of others.
Serfs: Those heavily indebted to banks. People who work paycheque-to-paycheque and live in fear of a missed payment to their Baron banker.
Thralls: The people who has lost their home to the Baron bankers, relying on social assistance, working a job for minimum wage while basic living expenses take up 100% of that paycheque. In some situations Thralls of today are 100% reliant on the welfare state to provide for them and their families. In some cases they cannot work because working becomes more expensive than not working--through the cost of commuting to work and putting kids in daycare.
Freemen: Those who have no or minimal debt but are not in a position "above" Serfs or Thralls. Freemen make no financial gain from the servitude of others but also have no indebtedness to the Barons. In some cases, their material possessions may be less than that of a Serf, who, under the support of the Barons still have some degree of provision. Though in the long run, the Serf pays back to their Baron much more than what he is given--through usury.
Knights: Those Freeman or Serfs, whom through entrepreneurship, gain some degree of wealth by their own means. Ideally with a venture that provides an increase in the quality of life of those around them, thereby differentiating themselves from the Barons, and achieving so without requiring the servitude of others nor future servitude in the form of usury.