Social Security beneficiaries who continue to work before they reach full retirement age are subject to what's known as the earnings test.  In 2015, if you make more than $15,720, you'll lose $1 in benefits for every $2 you earn over that limit.

In the year you reach full retirement age, a more lenient test applies: You'll give up $1 for every $3 in earnings over $41,880 before your birthday. Starting in the month you reach your full retirement age, you can earn as much as you want without worrying about the earnings test.

Only wages from a job or self-employment income will trigger the earnings test; investment income, pension benefits, and money withdrawn from an IRA or 401(k) aren't counted.

 
Courtesy of Kiplinger's