The IRS uses a tax identification number (TIN) to identify each taxpayer.
If your business is a single-owner LLC or a sole proprietorship without employees, you have the option of using your own social security number as your business TIN.
All other business types, however, are required to have a separate TIN, which is called an Employer ID Number (EIN). Sole proprietors and single-owner LLCs also have the option of getting an EIN if they choose.
You can apply for an EIN online, on the IRS website, pretty quickly and easily. Once you complete the application and the IRS validates it, you'll get your EIN right away. Be sure to save your EIN notice in a safe place (and a couple backups!), for reference and future use.