What is a Form W-8 used for?

A W-8 form is typically completed by a non-US resident who wishes to notify a request to tax exemption on US earner income.
There are several W-8 forms made available by the IRS, such as W-8 BEN, W-8 BEN-E, W-8 ECI, W-8 EXP, W-8 IMY and it is your duty to fill the correct form. However, forms and instructions are readily available on the IRS website.