From the summary within the report: "The windfall elimination provision (WEP) reduces the Social Security benefits of workers who also have pension benefits from employment not covered by Social Security. Its purpose is to remove an advantage or 'windfall' these workers would otherwise receive as a result of the interaction between the Social Security benefit formula and the workers' relatively small portion of their careers in Social Security-covered employment. Opponents contend the provision is imprecise and can be unfair."
CRS Report for Congress, 98-35
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html