Social security disabilities benefits come from FICA taxes taken out your check when you are paid. SSI benefits are basically a form of welfare there are pay to disable people whom have not pay enough FICA taxes or social disabilities taxes. So even if you have not paid enough social security disability taxes  if you are disable you may still be able to get  SSI benefits. Unfortunate SSI  benefits are sort of like food stamps in which you also have to prove that you are  pretty impoverished. So that is the principle difference between  SSI benefits  and social security disabilities benefits.