The IRS assigns employers, sole proprietors, corporations, partnerships, nonprofit associations, and other business entities a nine-digit Employer Identification Number, also known as an EIN or Federal Tax ID number.  If you form a C or S corporation, pay wages to at least one employee, or file pension or excise tax returns, you will have complete IRS Form SS-4 and get an EIN.  If you have a sole proprietorship with no employees, no pension plans, and no excise taxes, you will not have to get an EIN, but the sole proprietorship itself can use the social security number for income tax, self-employment, and other tax purposes.  Contact the IRS and get their publication, Understanding Your EIN, if you need further explanation.