Social Security encompasses several social welfare and social insurance programs and is funded through payroll taxes called Federal Insurance Contributions Act tax (FICA) and/or Self Employed Contributions Act Tax (SECA). Tax deposits are collected by the Internal Revenue Service (IRS) and are formally entrusted to various Social Security Trust Funds.
With a few exceptions all salaried income has a FICA and/or SECA tax collected on it. With few exceptions all legal residents working in the United States now have an individual Social Security number. Nearly all working (and many non-working) residents since Social Security's 1935 inception have had a Social Security number since it is required to do a wide range of things from paying the IRS to getting a job.
Income derived from Social Security is currently estimated to keep roughly 20% of all Americans, age 65 or older, above the Federally defined poverty level.