Income taxes are typically structured to be progressive taxes. For this reason, it is generally advocated by those who think that taxation should be borne more by the rich than by the poor, even to the point of serving as a form of social redistribution. Some critics characterize this tax as a form of punishment for economic productivity. Other critics charge that income taxation is inherently socially intrusive because enforcement requires the government to collect large amounts of information about business and personal affairs, much of which is considered proprietary and confidential.

The crucial invention permitting the reliable collection of high income taxes was direct withholding of taxes from payrolls by employers; this works because most people in modern societies are salaried workers. This reduces the perceived burden of the tax, because employees never handle the money. Direct withholding also discourages cheating, because it requires the collaboration of employers, and as there are fewer employers than employees, the government's enforcement efforts can be deployed more effectively. However, direct withholding also has some drawbacks: it puts part of the burden of processing taxes on the employer, and it also complicates matters when the employee is in a situation where he or she should pay significantly less or more than what is expected from its salary (because of tax-deductible expenses, or side revenues). Direct withholding is the method of collection of choice in most countries implementing income taxes, with the exception of France, where direct withholding is periodically discussed, but has so far not been implemented.

Where income tax is not collected at source, it may become easier to cheat by lying about one's affairs. The government may then require that employers report the amounts they pay to employees.

Income tax, in addition to income, generally takes into account a variety of factors. Certain expenses, such as work-related expenses, donations to charities etc..., may be tax-deductible: that is, they are subtracted from the taxable revenue. Investments in some impoverished areas or industrial sectors may be encouraged through tax breaks (reduced rates). Donations to charities may be partly subtracted from the tax, in an original form of subsidy. Because of various exemptions, rebates etc..., income tax codes tend to be complicated. In some countries such as the United States, individuals often hire the service of a tax accountant so as to find the best way to reduce their tax.

Income tax fraud is a problem in most, if not all, countries implementing an income tax. Either one fails to declare income, or declares nonexistent expenses. Failure to declare income is especially easy for non-salaried work, especially those paid in cash. Tax enforcement authorities fight tax fraud using various methods, nowadays with the help of computer databases. They may, for instance, look for discrepancies between declared revenue and expenses along time. Tax enforcement authorities then target individuals for a tax audit – a more or less detailed review of the income and tax-deductible expenses of the individual.

Income tax may be collected from legal entities (e.g., companies) as well as natural persons (individuals), although, in some cases, the income tax on legal entities is levied on a slightly different basis than the income tax on individuals and may be called, in the case of income tax on companies, a corporation tax or a corporate income tax.

Source: http://en.wikipedia.org