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. Using a social security number, personal data can be obtained, and can let a criminal use the information for purposes of defrauding the owner of that social security number. Often this involves stealing money or the identity of that SSN owner.