What is the purpose of a W-4 form?

The purpose of Form W-4, titled Employee's Withholding Allowance Certificate, is to help your employer withhold from your pay the correct federal income tax, based on Single or Married filing status and based on any dependents/allowance exemptions. If the correct amount is withheld, then you should either receive a refund or owe a reasonable, not a huge, amount when you file your tax return.