The Medical-Vocational Guidelines are used by Social Security to determine disability due to exertional impairments at step 5 of the sequential evaluation process.
The guidelines, or the "grids," consider a claimant's exertional level (that's the medical part) and the claimant's age, education and work history (the vocational factors). Depending upon these medical-vocational factors, the SSA determines that a person is either disabled or not disabled. The grids are potentially a factor in every Social Security disability case. When the grids to do not provide a definitive answer, vocational evidence is necessary.
I have added to this site a permanent page discussing the Medical-Vocational guidelines (the new page is listed on the sidebar, under Important Topics for Claimants). Please take a look!