Form W-4 is the IRS form that determines how much federal withholding tax will be deducted from your paycheck, at least from your salary. You complete it when you are hired. If you do not complete the form, your status for federal tax withholding will default to single. You want to submit a new Form W-4 whenever your personal or financial situation changes.
For more on Form W-4, see the related FAQ elsewhere on this website.