A taxpayer identification number, or TIN, is used by the Internal Revenue Service to track individuals and entities for tax purposes, and an employer identification number, or EIN, is one form of a TIN. Many people confuse a TIN with an individual taxpayer identification number, or ITIN, which is also a form of a TIN.
An EIN is used by the IRS to recognize a business. Businesses must submit a form requesting an EIN when they open so that any taxes paid while operating as the business can be tracked separately from personal taxes. An ITIN is for nonresident and resident aliens who cannot obtain a Social Security number. A TIN must be on any tax related documents sent to the IRS, including tax returns and treaty benefit claim forms.