Definition of 'W-9 Form'
An IRS form, also known as "Request for Taxpayer Identification Number and Certification", which is used by an individual defined as a "U.S. person" or a resident alien to verify his or her taxpayer identification number (TIN).
An entity that is required to file an information return with the IRS must obtain your correct TIN to report, for example, income paid to you, real estate transactions, mortgage interest you paid, etc. For example, companies that issue dividends use the W-9 form to verify a shareholder's TIN.
Investopedia explains 'W-9 Form'
If you are defined as a "foreign person" for federal tax purposes, you do not use the W-9 form. Rather, the IRS requires you to use the appropriate W-8 form.
Even though employees are legally required to supply certain personal information to their employers, an employee's privacy is protected by law. An employer that discloses an employee's personal information in any unauthorized way may be subjected to civil and criminal prosecution.