Why do I have estimated tax payments?

[robtm]

Estimated tax is the method used to pay tax on income that is not subject to withholding (for example, earnings from self-employment, interest, dividends, rents,alimony, etc.). In addition, if you do not elect voluntary withholding, you should make estimated tax payments on other taxable income, such as unemployment compensation and the taxable part of your social security benefits.