Although many people use the terms Social Security Disability Insurance (SSDI) and Supplemental Security Income (SSI) interchangeably, the two are actually different federal programs. SSDI is an insurance program funded by an individual’s payroll taxes. The SSI, on the other hand, is a need-based income supplement program funded by general tax revenues. However, both programs are managed by the Social Security Administration (SSA), and for both the SSDI and the SSI programs, a worker must be determined to be physically or mentally disabled and unable to work for at least one year.

The SSDI program provides benefits to the disabled or blind individuals who are considered "insured" because of their contributions to the Social Security trust fund. These contributions are the Federal Insurance Contributions Act (FICA) social security tax paid on the workers’ earnings.

According to the Social Security Administration, a person qualifies for SSDI, also called Social Security Disability Benefits or SSD, if they have a physical or mental condition that prevents them from working for at least twelve months or that will cause them to die. Eligible candidates must also be younger than 65 and have worked at least five out of the last ten years. Disabled people who qualify should receive SSDI until their condition improves. If their condition will not improve, SSDI is intended to be a guaranteed source of income for them.

The Supplemental Security Income program is a cash assistance program which is based on an individual’s financial need, not on an individual’s work history. The SSI is financed by general tax funds of the U.S. Treasury. The SSI program was set up to help the blind, disabled, and elderly people with little or no income by providing them with a monthly check to pay for food, clothing, and shelter.

In order to be eligible to receive SSI benefits, an individual must be physically or mentally disabled, blind, or be at least sixty-five years old. An eligible candidate must also have limited resources and income. Children who are blind or disabled are also eligible to receive SSI funds.

People who receive SSI are usually also eligible to receive monthly food stamps and Medicaid, which helps pay for doctor visits and hospital bills. The amount of SSI an individual can receive depends on where the person lives, what he or she owns, and the amount of monthly income the person brings in. Thus, SSI benefits are more limited than SSDI benefits.