SSI and SSDI are both for people with long term, severe impairments making it so they cannot earn above a certain amount a month (about $1000). SSI is for poor people who do not have enough work credits to get SSDI. SSDI is a disability insurance program that workers pay taxes to have. Some people get SSDI, but if they only worked low wage jobs or have a minimal number of work credits, they may not get as much as SSI would pay them, so the welfare program SSI makes up the difference. Since SSDI is insurance that taxpayers pay for, it doesn't matter what assets the claimant has or passive (non-wage) income, such as from investments. A person on SSDI working part time earning perhaps $600 a month will not lose their disability income or be docked. A person on SSI also will not lose their disability income for earning $600 a month, but for every $2 they make above a certain amount, they will lose $1 in benefits ($65? $85? It changes every year according to inflation). The SSA has a booklet called working while disabled that explains these things. I'll bet it's in PDF format on their website.