SSD is for people who have worked and earned enough quarters of coverage for disability income. SSI is a supplemental program for people who receive low SSD payments, or have not worked enough quarters for SSD coverage but are financially needy.
People who receive SSD may also get MEDICARE. Those who receive SSI are entitled to get MEDI-CAL. Some people are eligible to receive all of these benefits.
To be eligible for either SSD or SSI, you must be unable to work due to a physical or mental impairment or combination of impairments. You must show that your medical problems are expected to result in death or can be expected to last for at least 12 months.