Social Security Disability is the federal program, supported by payroll taxes, that pays benefits to people who cannot work due to a medical condition. In order to qualify, you must have a medical condition that meets Social Security's definition of disability and is expected to last at least one year or result in death.
You will be considered disabled if you can provide medical evidence that are unable to do the work that you did before and you cannot adjust to other work because of your medical condition(s). No benefits are payable for partial disability or for short-term disability.
Generally, to be eligible for disability benefits, you must have worked long enough and acquired enough work credits. The number of work credits required will depend on your age when you become disabled. There are different eligibility rules for children under the age of 18.
How Social Security determines if you are disabled
There are five steps along the path to being declared disabled http://www.lupus.org/webmodules/webarticlesnet/templates/new_resourcesgovernment.aspx?articleid=168&zoneid=38 for info