What is the difference between SSI and SSD?

The difference between SSD and SSI are the technical qualifications for those benefits beyond the requirement that you’re being disabled. For SSD, you have to have worked and paid FICA taxes, which buys you insurance to claim Social Security Disability benefits. For SSI, even if you haven’t worked, it’s a low income program and so if you find yourself in a situation where you have worked but not for a while and you don’t have any savings left and you are low income or you have not worked and you are low income you can qualify for SSI.