Work and Social Security retirement benefits

If you work and are full retirement age or older, you may keep all of your benefits, no matter how much you earn. If you are younger than full retirement age, there is a limit to how much you can earn and still receive full Social Security benefits. If you are younger than full retirement age during all of 2009, Social Security will deduct $1 from your benefits for each $2 you earned above $14,160. If you were born January 2, 1943, through January 1, 1955, then your full retirement age for retirement insurance benefits is 66.

See this reference :
http://www.ssa.gov/pubs/10069.html

Also see this calculator which will estimate reduction in benefits based on your age and earnings.
http://www.ssa.gov/OACT/COLA/RTeffect.html