This is the wording from the SSA site can't get any simpler than this:

http://ssa-custhelp.ssa.gov/app/answ...i%20and%20ssdi

Thanks,
Looks like public domain so we can post it here.
[As a public agency, we generally don't own rights to material on our website. (Almost all of the material consists of new or republished government documents and as such, it is considered public domain.)]


Difference between Social Security disability and SSI disability

[Social Security is responsible for two major programs that provide benefits based on disability:
Social Security Disability Insurance (SSDI), which is based on prior work under Social Security,
and
Supplemental Security Income (SSI).
Under SSI, payments are made on the basis of financial need.

Social Security Disability Insurance (SSDI) is financed with Social Security taxes paid by workers, employers, and self-employed persons. To be eligible for a Social Security benefit, the worker must earn sufficient credits based on taxable work to be "insured" for Social Security purposes. Disability benefits are payable to blind or disabled workers, widow(er)s, or adults disabled since childhood, who are otherwise eligible. The amount of the monthly disability benefit is based on the Social Security earnings record of the insured worker.

Supplemental Security Income (SSI) is a program financed through general revenues. SSI disability benefits are payable to adults or children who are disabled or blind, have limited income and resources, meet the living arrangement requirements, and are otherwise eligible. The monthly payment varies up to the maximum federal benefit rate, which may be supplemented by the State or decreased by countable income and resources. ]

posted info is from the same link as given by previous poster -
http://www.socialsecurity.gov/websitepolicies.htm

Thanks Jo*mar, that pretty much somes up the question I had.