A Social Security number (SSN) is a nine-digit number that the U.S. government issues to all U.S. citizens and eligible U.S. residents who apply for one. The government uses this number to keep track of your lifetime earnings and the number of years worked.
When the time comes to retire, or if you ever need to receive Social Security disability income, the government uses the information about your contributions to Social Security to determine your eligibility and calculate your benefit payments. Most people will use the same Social Security number their entire lives, though some people might need to apply for a replacement number at some point because of identity theft.
