Taxpayer Identification Number (TIN)
A TIN is a unique number the IRS uses to identify you for tax administration purposes.
The most common TINs are the Social Security Number and the Employer Identification Number (EIN).
Your Social Security Number goes on your personal income tax return (IRS Form 1040).
The EIN is a unique number that identifies a Business Entity to the IRS. Unless you operate your business as a Sole Proprietorship and you have no employees, you probably need an EIN. An EIN is also called a Federal Tax Identification Number.1
1 See http://www.irs.gov/businesses/small/article/0,,id=98350,00.html, retrieved 7/23/11.