EIN: Employer Identification Number
An employer identification number (EIN) is a nine-digit number assigned by the IRS. It's used to identify the tax accounts of employers and certain others who have no employees. The IRS uses the number to identify taxpayers who are required to file various business tax returns. EINs are used by employers, sole proprietors, corporations, partnerships, non-profit associations, trusts, estates of decedents, government agencies, certain individuals, and other business entities. For more on EINs check out the IRS' definition of them here.
IRS: Internal Revenue Service
The IRS is a bureau of the U.S. Department of Treasury that helps taxpayers meet their tax responsibilities and enforces tax law. We interact with the IRS when helping companies get their EIN and companies interact with the IRS for any necessary tax filings. For more on the IRS check out their about page here.
LLC: Limited Liability Corporation
A limited liability company is the U.S.-specific form of a private limited company. It is a business structure that can combine the pass-through taxation of a partnership or sole proprietorship with the limited liability of a corporation. For more on LLCs check out the IRS' tax topic post on them here.
SSN: Social Security Number
The U.S. SSA (Social Security Administration) issues an SSN in order to identify a US Citizen, Permanent Resident, or Temporary Nonimmigrant Worker.
ITIN: Individual Taxpayer Identification Number
The IRS issues an ITIN to people who have a U.S. tax filing (or informational reporting requirement) and are not eligible for an SSN. For more on ITINs check out the IRS' definition of them here.