Tell me about social security numbers?

In the United States, a Social Security number (or SSN) is a 9 digit number issued to citizens, permanent residents, and temporary (working) residents under section 205(c)(2) of the Social Security Act, codified as 42 U.S.C. § 405(c)(2). The number is issued to an individual by the Social Security Administration, an agency of the federal government. Its primary purposes are to track individuals' Social Security benefits, and to track individuals for taxation purposes. In recent years the SSN has become a de facto national identification number. A social security number may be obtained by applying on Form SS–5, "Application for A Social Security Number Card" (see 20 C.F.R. sec. 422.103(b).