Money isn't allowed in the prison. Families can deposit money into an inmate's account which can be used in the prison commissary, the place where inmates can buy things.The products purchased from the commissary then become used in the prison economy.
Without actual money the paper money used in the prison are stamps.
What's intrigued me about stamps, as I've asked questions about the prison economy, is how their value might be inflated or deflated. How much is a stamp worth?
While it's been hard to nail the specifics down from the guys in the Monday night Bible study, it does appear that the basic laws of supply and demand affect the value of stamps. To explore this more see this essay written by a Texas inmate, "In Prison, Stamps Are the Currency That Binds Everyone."