Center on Budget and Policy Priorities, Where Do Federal Tax Revenues Come From?:

In fiscal year 2011, the federal government spent $3.6 trillion on the services it provides, such as national defense, health care programs like Medicare and Medicaid, Social Security benefits for the elderly and disabled, and investments in infrastructure and education, in addition to interest on the debt. … Of that $3.6 trillion, $2.2 trillion was financed by federal tax revenues and $83 billion by excess profits on assets held by the Federal Reserve.  (The remaining $1.3 trillion was financed by borrowing.) … The three main sources of federal tax revenue are individual income taxes, payroll taxes, and corporate income taxes; other sources of tax revenue include excise taxes, the estate tax, and other taxes and fees.