The U.S. Treasury divides all federal spending into three groups: mandatory spending, discretionary spending and interest on debt. Mandatory and discretionary spending account for more than ninety percent of all federal spending, and pay for all of the government services and programs on which we rely. Interest on debt, which is a much smaller amount than the other two categories, is the interest the government pays on its accumulated debt, minus interest income received by the government for assets it owns. The pie chart shows federal spending in 2015 broken into these three categories.
The mandatory expenditures are monies that the federal government has to spend. The largest one is retirement i.e. social security. Because retired people previously paid the government through social security tax, the government must pay them back when they retire. Heath care is another - if the government does not help the people in need, they will die. The third main mandatory expenditure, as the other helper has explained, is payments on debts that we owe other countries.
