W-8 Form

An IRS form that grants a foreigner an exemption from certain U.S. information return reporting and backup withholding regulations.

There are many variations of the W-8 form, such as the W-8BEN and W-8ECI.
One of the main purposes of this document is to show brokers and mutual fund companies that the foreign investor is not subject to the typical taxation practice where tax is withheld from investment income (e.g. dividends and coupon payments). If adequate documentation is not supplied with a W-8 form, the foreign investor will be subject to the normal rates of backup withholding.