A W-4 tax form is an IRS document that employees fill out and submit to their employers to determine how much federal income tax should be withheld from their paychecks. The form is formally titled "Employees Withholding Certificate". Accurately completing the W-4 form can help employees avoid overpaying their taxes throughout the year or owing a large balance at tax time. The form includes personal information such as name, address, and Social Security number, as well as information about filing status, dependents, and tax credits. Employees may need to submit a revised W-4 if their situation changes, such as getting married or divorced, having a child, or picking up a second job.