Mandatory spending refers to funds whose appropriation is not directly controlled by Congress through the annual budget-making process. It is spending which is, by law, obligated automatically.
Social Security outlays cannot be changed without changing the law. This is mandatory, by definition . Interest payments on the public debt are a part of the annual Congressional discretionary budget outlays. This is voluntary, again by definition. The United States is under no legal obligation to honor its debts, anymore than, say, Bank of America is - if it were, it would distort the marketability of that debt. It is under contractual obligation, like any other borrower, but since the US has never actually tried not paying its debts, I don't think anybody really knows what consequence this would have for Congress if we decided to go that route. In practice, its conjecture in the extreme.
Unemployment benefits, most veterans benefits, income support programs, some product subsidies. Any spending that is required by law, and not appropriated annually by Congress.
It's actually pretty surprising how much of the budget is mandatory - this is why reducing government expenses is so difficult. The budget can be passed with relative ease, bypassing most of the procedural obstacles that slow down legislation, and is a consequence a pretty volatile outlay. It grows and shrinks pretty dramatically over a relatively short time, and is difficult to predict forward. Mandatory outlays, on the other hand, change little (speaking fundamentally) over time and are pretty easy to predict forward - it's just a matter of statistics once you assume the law doesn't change.
Something like 3/4 of the USDA budget is mandatory, for example.
