A SSN is a (TIN) Taxpayer Identification number. All US citizens must have a SSN to work and file taxes.
A Taxpayer Identification Number (TIN) is an identifying number used for tax purposes in the United States. Types of TIN's:
SSNs are used by individuals who have the right to work in the United States.
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.
EINs are used by employers, sole proprietors, corporations, partnerships, non-profit associations, trusts, estates of decedents, government agencies, certain individuals, and other business entities