If you qualify for Supplemental Security Income (SSI), a need-based disability benefit program, you will be entitled to receive benefits as far back as your date of application, as long as you meet the income and resource limitations of the program, in addition to the medical qualifications. To be eligible for SSI you must be earning less than $1,000 per month because of a disability, and you cannot have more that $2,000 in savings, retirement, and non-essential items. SSI is a last-resort benefit. It pays only after you receive everything else for which you are eligible
If you qualify for Social Security Disability Insurance (SSDI), you will be entitled to benefits as far back as your date of application and potentially even up to 12 months retroactive to this if your disability stopped you from working before you applied for benefits. To be eligible for SSDI you must be earning less than $1,000 per month because of a disability. There is no limit of how much money you have saved or what you own. SSDI is a set amount based on how much you've paid into the Social Security fund (sometimes listed as FICA on your paychecks).
Some people get both SSI and SSDI if their SSDI amount is lower that the SSI payment amount for their state.
Some people receive SSI benefits under the Presumptive Disability rule while waiting for the Social Security Administration (SSA) to decide if you are eligible. This is possible when a disability is fairly obvious such as blindness, deafness, more severe types of cancers, spinal cord injuries, etc. In presumptive awards only SSI payments are made. If later the SSA decides you are eligible for SSDI, they will subtract the SSI you were paid from your SSDI back pay.