As the name implies, this number is a nine-digit number used as a tracking number by the IRS for tax purposes. Whereas Social Security numbers are in the format of XXX-XX-XXXX, tax ID numbers are read as XX-XXXXXXX. This number is required information on all tax returns filed with the IRS. Tax ID numbers come in three different forms. Individuals are assigned TINs in the form of Social Security numbers (SSNs), whereas businesses such as corporation and partnerships are assigned Employer identification numbers (EINs). Trusts, fiduciaries and other non-business entities are assigned straight tax ID numbers (TINs).