A social security card contains a nine-digit number, which is issued by the government to all US citizens and eligible residents who apply. With this reference, a track record is maintained for the number of years you worked and your overall income. At the time of retirement, the government uses this information to decipher your eligibility and calculate your benefits. Most people will use the same number their entire lives, but some might need to buy a social security card due to identity theft. It is more than just a retirement program since it provides important disability insurance and life insurance protection as well.