Social Security - Reduction for under full retirement age

If you are under full retirement age (ages 66-67 depending on date of birth) for the entire year, SSA deducts $1 from your benefit payments for every $2 you earn above the annual limit. For 2013, that limit is $15,120.
In the year you reach full retirement age, SSA deducts $1 in benefits for every $3 you earn above a different limit, but SS only counts earnings before the month you reach your full retirement age. If you will reach full retirement age in 2013, the limit on your earnings for the months before full retirement age is $40,080.
Starting with the month you reach full retirement age, you can get your benefits with no limit on your earnings.