"The largest source of revenue for the federal government is the individual income tax. The federal individual income tax is levied on an individual's taxable income, which is adjusted gross income (AGI) less deductions. Tax rates based on filing status (e.g., married filing jointly, head of household, or single individual) determine the amount of tax liability. Income tax rates in the United States are generally progressive, such that higher levels of income are typically taxed at higher rates. Once tentative tax liability is calculated, tax credits can be used to reduce tax liability. Tax deductions and tax credits are tools available to policymakers to increase or decrease the after-tax price of undertaking specific activities. Individuals with high levels of deductions and credits relative to income may be required to pay the alternative minimum tax (AMT)."
CRS Report for Congress, R45145