An income tax is a tax levied on the financial income of persons, corporations, or other legal entities. Various income tax systems exist, with varying degrees of tax incidence. Income taxation can be progressive, proportional, or regressive. When the tax is levied on the income of companies, it is often called a corporate tax, corporate income tax, or profit tax. Individual income taxes often tax the total income of the individual (with some deductions permitted), while corporate income taxes often tax net income (the difference between gross receipts, expenses, and additional write-offs).

Principles

The "tax net" refers to the types of payment that are taxed, which included personal earnings (wages), capital gains, and business income. The rates for different types of income may vary and some may not be taxed at all. Capital gains may be taxed when realized (e.g. when shares are sold) or when incurred (e.g. when shares appreciate in value). Business income may only be taxed if it is significant or based on the manner in which it is paid. Some types of income, such as interest on bank savings, may be considered as personal earnings (similar to wages) or as a realized property gain (similar to selling shares). In some tax systems, personal earnings may be strictly defined where labor, skill, or investment is required (e.g. wages); in others, they may be defined broadly to include windfalls (e.g. gambling wins).

Tax rates may be progressive, regressive, or flat. A progressive tax taxes differentially based on how much has been earned. For example, the first $10,000 in earnings may be taxed at 5%, the next $10,000 at 10%, and any more income at 20%. Alternatively, a flat tax taxes all earnings at the same rate. A regressive income tax may tax income up to a certain amount, such as taxing only the first $90,000 earned. A tax system may use different taxation methods for different types of income. However, the idea of a progressive income tax has garnered support from economists and political scientists of many different ideologies, from Adam Smith in The Wealth of Nations to Karl Marx in ''The Communist Manifesto.

Personal income tax is often collected on a pay-as-you-earn basis, with small corrections made soon after the end of the tax year. These corrections take one of two forms: payments to the government, for taxpayers who have not paid enough during the tax year; and tax refunds from the government for those who have overpaid. Income tax systems will often have deductions available that lessen the total tax liability by reducing total taxable income. They may allow losses from one type of income to be counted against another. For example, a loss on the stock market may be deducted against taxes paid on wages. Other tax systems may isolate the loss, such that business losses can only be deducted against business tax by carrying forward the loss to later tax years.