What it is:
A W-4 form is a standard IRS form an employee uses to report federal taxability status to an employer.
How it works/Example:
An employer is required by law to withhold taxes from an employee's pay based on information reflected in the employee's W-4 form. The employee uses a W-4 to report his or her marital status, spousal employment status, any physical disabilities and the number of qualifying dependents (for example, children). If the employee is married, the W-4 form also asks whether the employee plans to file income taxes separately or jointly with his or her spouse.
Why it matters:
Since an employer withholds taxes based on the information contained in a W-4, it is important that an employee resubmit an updated W-4 whenever he or she experiences significant changes in taxable status (for example, marriage, divorce or the birth of a child).