http://www.ssa.gov/pubs/10035.html
Social Security Administration
Because Social Security is part of the retirement plans, this booklet was written to help you make sensible decisions about your retirement to include SSI. You'll find explanations of how to qualify for Social Security benefits, how your earnings and age affect your benefits, and how to choose your retirement date. You'll also find information about Social Security's future and where your Social Security tax dollars go.