The main difference between SSI and SSDI lies in their eligibility criteria and funding sources. SSI pays benefits to aid aged, blind, and disabled individuals who have limited income and resources, regardless of their work history. It is funded from general tax revenues. On the other hand, SSDI is available to individuals who have a sufficient work history and have paid into the Social Security system through payroll taxes. SSDI benefits are essentially insurance that workers earn through their contributions.