While U.S. income tax law is very complex, the underlying idea is relatively easy to understand. Simplifying greatly, gross income is all income from all sources (§ 61) less any exclusions (§ 101 et seq.). An exclusion is something that Congress has effectively said a taxpayer need not include in his or her income for tax purposes, such as employer-paid health insurance (§ 106) or interest from tax-exempt bonds (§ 103). Exclusions, often referred to as deductions, are a matter of legislative grace; that is, taxpayers may not exclude, or deduct, from gross income any item which Congress has not specifically allowed.

For individuals, Adjusted Gross Income (AGI) is gross income less any above-the-line deductions (§ 62). Above-the-line deductions are listed in § 62 and include trade or business deductions, alimony (§ 215), and moving expenses (§ 217). Taxable income is AGI less (1) itemized deductions or the applicable standard deduction, whichever is greater, and (2) a deduction for any allowable personal exemptions for the taxpayer, the taxpayer's spouse (if filing jointly), and the taxpayer's dependents. (In certain cases involving higher income taxpayers, the allowed personal exemptions may be reduced or even eliminated.)

Non-itemizers take the standard deduction. Itemized deductions include any deduction not listed in § 62 such as charitable contributions (§ 170) and certain medical expenses (§ 213). Taxable income is then multiplied by the appropriate tax rate to arrive at the tax due. Tax credits such as the Earned Income Tax Credit (§ 32) or the Child Tax Credit (§ 24) lower the tax owed on a dollar-for-dollar basis. This means tax credits are more valuable than deductions of the same amount, because deductions are applied before the tax rate, while credits are applied after. For instance, with a 35% tax rate, a deduction of $100 would save only $35 of taxes, while a $100 credit would save $100 worth of taxes.

Types of income

For tax purposes, income can be divided in a variety of ways. The first division is between ordinary income and capital gains. Ordinary income includes compensation for personal services such as wages and salaries, business profit, dividends from stock shares, and interest income from invested funds while capital gain generally comes from the sale of investment property. Congress has typically shown a preference for long-term investment by having a capital gains tax rate lower than the ordinary income rate. However, only long-term capital gains get preferential treatment; short-term capital gains (from property held for one year or less) are taxed at the same rate as ordinary income. Added complications come from various distinctions within each category. For instance, qualified dividends, which were previously taxed at ordinary income rates (as non-qualified dividends currently are), can be currently taxed at long-term capital gain rates until 2011 under the Jobs and Growth Tax Relief Reconciliation Act of 2003, and within long-term capital gains, gains on certain real estate, collectibles, and small business stock each have their own tax rates. The rules for offsetting capital losses with gains (whether capital or ordinary) add further complications. In ordinary usage, when someone speaks of their "tax rate", they typically are referring to their marginal tax rate for ordinary income.

Another important distinction in types of income is income from passive activities versus non-passive activities (§ 469), an attempt to curb tax shelters used by taxpayers not directly involved with an activity other than as an investor ("passive").