I tried to find the thread where someone was asking the difference. Here is the answer. The main difference between Social Security Disability (SSD, or SSDI) and Supplemental Security Income (SSI) is the fact that SSD is available to workers who have accumulated a sufficient number of work credits, while SSI disability benefits are available to low-income individuals who have either never worked or who haven't earned enough work credits to qualify for SSD.
While many people don't distinguish between SSI (Supplemental Security Income) and SSDI (Social Security Disability Insurance), they are two completely different governmental programs. While both programs are overseen and managed by the Social Security Administration, and medical eligibility for disability is determined in the same manner for both programs, there are distinct differences between the two.