One key difference between W-9 and W-8 forms is that W-9 forms are used by U.S. citizens and resident aliens to submit their taxpayer identification number (TIN)  to employers or other entities.In contrast, W-8 forms are used by foreign individuals or entities who are not U.S. citizens or residents but have earned income or worked in the U.S. These forms help them claim tax treaty benefits and manage U.S. tax withholding.