Social Security Number, known also as SSN, is a nine-digit government-issued number for U.S. citizens and permanent residents (exceptions apply). The number is issued to an individual by a federal agency, the Social Security Administration.
Originally, SSN was created to track individuals for social security reasons. However, over time, it has basically become the national identification number for tax purposes and other purposes as well. For instance, SSN can be used for opening a bank account, applying for personal credit, insurance, medical coverage, and so on.