SSD
This program is financed with Social Security taxes paid by workers, employers, and self-employed persons. Social Security Disability (SSD) benefits are payable to individuals who have been found blind or disabled under the Social Security Administration's rules. To be eligible for SSD benefits, you must earn sufficient credits based on taxable work to be "insured" for Social Security purposes. If found disabled, you will get Medicare coverage automatically after receiving disability benefits for two years. The amount of disability benefits received is based on your Social Security earnings record.
SSI
This program is financed through general revenues from taxes, meaning benefits are not based on your prior work history. Supplement Security Income (SSI) benefits are payable to individuals age 65 or older, and individuals who are disabled or blind. In order to be eligible for SSI benefits, you must have limited income and resources that meet the living arrangement requirements AND be a U.S. citizen or national, or in one of certain categories of aliens. If awarded SSI benefits, the amount of benefit varies up to the maximum federal benefit rate, which may be supplemented by the state. If SSI benefits are awarded, you will automatically be eligible for Medicaid (in most states).