A W-9 form from the Internal Revenue Service is used to request a taxpayer identification number. One common use for the W-9 is to obtain the payee's taxpayer information from independent contractors or freelancers. It's used to get the TIN for other kinds of financial transactions as well.
Request for Contractor's TIN and Certification
A company uses this form when it needs a tax identification number from an individual or business to report payments to the IRS. It isn't used to report the payments themselves. If you're self-employed and contract with a company to provide a service, you'll need to submit a W-9 to give the company your TIN. This identification number can be your Social Security number or individual taxpayer identification number. The ITIN identifies resident aliens for tax purposes and is an alternative for contractors who prefer to not to give out their Social Security number. Likewise, if your business pays contractors -- people you pay to perform work or provide a service who aren't employees -- you'll use this form.
Other Uses and Alternative
The W-9 form also serves to obtain the TIN for deals involving real estate, acquiring or abandoning secured property and debt cancellation. It's needed as well by companies who report individual retirement account payments or interest paid on a mortgage to the IRS. A substitute form, such as a contract or document a business generates that meets the IRS requirements, can be used instead of a W-9.