SSD is for people who have a work history. Meaning the person has worked for 5 out of the past 10 years before the illness/injury occurred at the levels required to get credit at the SSA. You get one credit for each quarter you work. Work credit expires for each quarter you do not work.
The rest of the requirements are the same for both SSD and SSI claims. You must have an illness or injury that is medically determinable that prevents you from working full time that has or will last for at least 12 months or will end in death.
SSD claims usually result in more money monthly than SSI claims.
SSI is for anyone who, for any reason, does not have a work history, including children who have a medically determinable impairment that has or will last for at least 12 months or end in death.
SSI is a needs-based program. This means if you have too many assets, too much money in the bank, retirement accounts, or other property or income, you most likely will not qualify for this benefit.
Similarly, if you are disabled and do not have a work history, but your spouse is working, you most likely will not qualify for these benefits. Household income is used to calculate SSI.
SSI claims have a cap that is determined each year. The SSA does reduce the monthly amount for various reasons, including but not limited to assistance from others, living with others, receiving income from a part-time job, and for a variety of other reasons as well.