A document published by the Internal Revenue Service that outlines the process for withholding and estimated tax that individuals must follow in order to pay their taxes. Typically, workers have taxes withheld from their paychecks. Workers who do not have taxes withheld from their income – whether due to employment status or for another reason – will have to pay estimated taxes over the course of the year. Income tax is not paid like a typical bill for a good or service. Instead of money coming due once a service is performed or good purchased, it is due when income is earned. Taxpayers who do not pay enough tax during the course of the year may wind up paying a penalty for each payment period.