Debt
It is a commodity that one receives goods or services (called a "Principal Amount") today in exchange for paying in goods or services at a higher corresponding value than the principal amount at a predetermined later time. This difference is called "interest" and represents the main source of profit by which debt-issuing entities earn profit. The principal amount and the interest are priced in a pre-agreed upon currency.
In other words, the true product is not the goods or service provided, but the promise of future payment of the principal amount and its corresponding interest. This allows debt to be transferred between debt-issuing entities, usually banks, allowing them to both profit off of the reputation of the debtor (if the debtor is trustworthy) and offload the risk of default on another entity (if the debtor is not trustworthy or if the economy is currently unstable).
One can easily see how this would create an economic class that has power over society via control of the money supply, as well as a system or set of norms for determining a person's trustworthiness. The latter's effect on a given society's culture is apparent, in that the image of debt and commodification would begin to color and cloud interactions between individuals.