A Taxpayer Identification Number or TIN is an identifying number used for tax purposes in the United States. It is also known as a Tax Identification Number or Federal Taxpayer Identification Number. It may be assigned by the Social Security Administration or by the IRS (Internal Revenue Service.)
A TIN may be
SSNs are used by individuals who have the right to work in the US.
ITINs are used by aliens who may or may not have the right to work in the US, such as aliens on temporary visas and non-resident aliens with US income. Often the reason why people need the identification number is because they have been advised by a bank that no account will be opened without it, even if it is requested by a non-profit organization.
EINs are used by employers, sole proprietors, corporations, partnerships, non-profit associations, trusts, estates of decedents, government agencies, certain individuals, and other business entities.