The W-4 form is a document used by employees in the United States to inform their employer about their tax withholding preferences. By filling out this form, employees can indicate how much federal income tax should be withheld from their paychecks based on their personal financial situation, including factors like marital status and number of dependents. Employers use the information provided on the W-4 form to calculate the appropriate amount of tax to withhold, ensuring that employees do not owe a large sum at tax time. It is important for employees to review and update their W-4 form whenever their financial situation changes, such as after a marriage or the birth of a child.