Although both programs use the same basic definition of "disability," there are fundamental differences between the programs. The benefits and qualifying requirements differ, because SSI includes a component of financial need and can be obtained by those who are disabled but did not pay into the government system.
SSD, on the other hand, is the program that pays monthly benefits to people who are disabled and have paid into the system through working and paying their taxes. Many people confuse these two programs, which is easy to do because one person can receive payments from both programs. For example, if you worked for 20 years, became disabled, but you worked at a low paying job, your monthly benefit amount under SSD may be low enough that you also qualify for some SSI money, depending on the state in which you live.
The main thing to understand about the programs, is that when you apply for disability benefits with the SSA, you are applying for both types of benefit. The SSA will determine if you are not eligible financially for SSI. They will also determine, under their rules, if you are eligible and are disabled for SSD. Eligibility for benefits begins when you apply for benefits. If you do not apply, you are not eligible and you cannot go back in time and receive past benefits. Many people think that if they apply and they haven't worked for the past 5 years due to disability, if their application is approved the SSA will pay them for the entire time they have been off work. That is simply not true. SSI begins on the date of application, if one is found disabled. Under the SSD application, the most a person can receive in past due benefits is payments one year prior to their application date, as long as they were not working and they are found disabled for that past period of time.
The important thing to understand from this information is that no matter what type of benefit you are eligible to receive if you are disabled, you can receive nothing unless you apply. Therefore, if you become disabled and are no longer working, the sooner you apply, the better off you will be.