A tax identification number (TIN) is a unique identifier assigned to individuals and businesses for tax purposes. In the United States, the most common types of TINs include the Social Security Number (SSN) for individuals and the Employer Identification Number (EIN) for businesses. These numbers help the Internal Revenue Service (IRS) track tax obligations and ensure compliance with tax laws. TINs are essential for filing tax returns, opening bank accounts, and applying for loans. They help prevent identity theft and fraud by ensuring that each taxpayer is accurately identified. Individuals and businesses must keep their TINs secure to protect their financial information.