A taxpayer identification number is often referred to as an Employer Identification Number (EIN). It is a nine digit number that is used for taxes, banking and other business purposes. To obtain an EIN you need to obtain, complete and file the Form SS-4, "Application for Employer Identification Number" with the IRS. The form has a set of instructions on the back, which explains what parts of the form need to be completed depending on the reason for filing. You can obtain this form and download it from the IRS website at www.irs.gov. The form is free.