Employer Identification Number (EIN)
An EIN is also known as a federal tax identification number, and is used to identify a business entity . Employers with employees, business partnerships, and corporations and other types of organizations, must obtain an EIN from the IRS. The EIN is also known as an Employer Tax ID and Form SS-4.
You are required to have an EIN if you answer “yes” to any of the following six questions:
Do you have employees?
Do you operate your business as either a corporation or partnership?
Do you file any of these tax returns: employment, excise, or alcohol, tobacco and firearms?
Do you withhold taxes on income, other than wages, paid to a non-resident alien?
Do you have a Keogh plan?
Are you involved with any of the following types of organizations?
Trusts (except certain grantor-owned revocable trusts, IRAs, Exempt Organization Business Income Tax Returns),
Real estate mortgage investment conduits,
If you do need an EIN, use one of these four methods to apply:
The Internet EIN  application is the preferred method for customers to apply for and obtain an EIN. Once the application is completed, the information is validated during the online session, and an EIN is issued immediately. The online application process is available for all entities whose principal business, office or agency, or legal residence (in the case of an individual), is located in the United States or U.S. Territories. The principal officer, general partner, grantor, owner, trustor etc. must have a valid Taxpayer Identification Number (Social Security Number, Employer Identification Number, or Individual Taxpayer Identification Number) in order to use the online application. www.irs.gov , search: online EIN, http://www.irs.gov/businesses/small/article/0,,id=97860,00.html 
Apply by EIN toll-free at 800-829-4933
Taxpayers can obtain an EIN immediately by calling the Business & Specialty Tax Line at (800) 829-4933. The hours of operation are 7:00 a.m. - 7:00 p.m. local time, Monday through Friday. An assistor takes the information, assigns the EIN, and provides the number to an authorized individual over the telephone. Note: International applicants must call (267) 941-1099 (Not a toll-free number).
Apply By FAX
Taxpayers can FAX the completed Form SS-4  (PDF) application to their state FAX number (see Where to File - Business Forms and Filing Addresses ), after ensuring that the Form SS-4 contains all of the required information. If it is determined that the entity needs a new EIN, one will be assigned using the appropriate procedures for the entity type. If the taxpayer's fax number is provided, a fax will be sent back with the EIN within four (4) business days.
Apply By Mail
The processing timeframe for an EIN application received by mail is four weeks. Ensure that the Form SS-4  (PDF) contains all of the required information. If it is determined that the entity needs a new EIN, one will be assigned using the appropriate procedures for the entity type and mailed to the taxpayer. Find out where to mail Form SS-4 on the "Where to File Your Taxes" (for Form SS-4)  page.
Other Important Information
Daily Limit of EINs
Due to a high volume of requests for EINs, the IRS will begin limiting the number of EINs assigned per day to a responsible party . Effective April 11, 2011, a responsible party will be limited to five (5) EINs in one business day. This limit is in effect whether you apply online, by phone, fax or mail.
In order to identify the correct individuals and entities applying for EINs, language changes have been made to the EIN process. Refer to Responsible Parties and Nominees  to learn about these important changes before applying for an EIN.
Third Party Authorization
The Third Party Designee section must be completed at the bottom of the Form SS-4. The Form SS-4 must also be signed by the taxpayer for the third party designee authorization to be valid. The Form SS-4 must be mailed or faxed to the appropriate service center. A third party designee may call for an EIN; however a faxed Form SS-4, with the taxpayer's signature, is still required.
IRS assistors will take the information over the phone from the third party designee and ask the third party to fax the completed Form SS-4 to them (to the IRS assistor's attention) at an administrative fax number. After receiving the faxed Form SS-4, the EIN will be assigned and faxed back to the third party designee, or given over the phone. The third party designee's authority terminates at the time the EIN is assigned and released to the designee.