A federal tax ID number, or an employer identification number, is a number used to identify business entities, the IRS says. This number is needed when businesses file withholding certificates, tax returns and documentation. Some trusts and estates need an EIN as well.
Most businesses can apply online for an EIN, the IRS notes. After submitting the application, the EIN is automatically provided to the applicant. Businesses can also apply for an EIN by submitting Form SS-4 via fax or through regular mail. If a business loses its EIN, it can be reclaimed by phoning the IRS business and speciality tax line, the U.S. Small Business Association explains.