Answer:
Form W-9 is Request for Taxpayer Identification Number and Certification. It's a form that you fill out for a person or business who's required to file information about money that you paid to them (mortgage interest, IRA contributions, etc.) or that was paid to you (income, etc.). Form W-9 verifies for the Requestor that your correct taxpayer identification number (TIN), such as social security number or employer identification number (EIN) is correct for the information being reported to the IRS.
You complete the form and return it to the Requestor. You don't file a copy with your tax return. The Requestor will provide you with the forms you need for tax return, such as 1099-MISC (Miscellaneous Income), 1098 (Mortgage Interest Statement), etc.