Income tax basics 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 or interest from tax-exempt bonds (§ 103). 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, 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.