SSDI refers to Social Security Disability insurance, often called "SSD" or Social Security disability benefits. There are a variety of names and acronyms, but they all refer to the same thing. SSI, or Supplemental Security Insurance, is very similar to SSD, but people qualify for SSI under different terms. This is the biggest factor that differentiates between SSD and SSI.
If you are looking to get Social Security disability benefits, then you have to contribute to the system through taxes paid during your employment. More specifically, you have to have contributed for 20 quarters of coverage in the last ten years. Your disability must also qualify under Social Security Administration rules, and there are other provisions that dictate if someone can get SSD benefits.
SSI, on the other hand, is a little more lenient. You still have to qualify under specific rules, but if you didn't contribute enough through taxes to qualify for SSDI, then you could qualify under SSI. You must be at least 65 years old to get SSI benefits. If you get these benefits, the amount you get will depend on your monthly income and your resources. There is also a maximum rate for benefits.
No matter if you are going for Social Security disability benefits or Supplemental Security Income, the process can be tricky. If you are experiencing trouble with your claim or case, then it may be best to consult with an experienced attorney to help you. With so much at stake, you shouldn't leave it to chance.
Source: FindLaw, "What is the Difference Between SSDI and SSI?," Accessed March 4, 2016