A W-9 form is an IRS (Internal Revenue Service) tax form that is used to report payments, confirm a person’s name, address, and taxpayer identification number (TIN) for employment or other income-generating purposes. This form acts as an agreement that you, as a freelancer, are responsible for withholding taxes from your income. An W-8 BEN Form is the equivelant for non-US Citizens, being paid by a US Entity.
You are required to fill out this legal form in order to get paid.