EMPLOYER IDENTIFICATION NUMBER (EIN)
An EIN, your federal tax ID number, is a nine-digit number assigned to sole proprietors, corporations, partnerships, estates, trusts, and other entities for tax filing and reporting purposes and to establish your business tax account.
All businesses except sole proprietorships without employees must complete an interview-style application to obtain an EIN.  Go to this link, for more information and for an application:  http://www.irs.gov/businesses/small/article/0,,id=102767,00.html