SSI is Supplemental Security Income.It is given to people who are disabled but don't have enough work credits to qualify for disability through Social Security.And I believe it is usually given through the welfare office.. SSD is Social Security Disability.It is given to people who have enough work credits paid into the Social Security system.You have had to pay enough into it to qualify.. I totally understand what your friend is going through.I have been receiving SSD for a few years.People always assume I am getting welfare.My check is based on what I made during the 20+ years that I worked.. SSI checks are usually smaller than SSD checks,unless you didn't earn much money while you worked.Either way,it isn't enough to live off of.