FERS is a three-tiered retirement plan that consists of:
Basic Benefit Plan
Social Security
Thrift Savings Plan (TSP)
Deductions are automatically withheld from your salary for the Basic Benefit and Social Security.
Two of the three parts of FERS (Social Security and TSP) are portable and can go with you if you leave the federal government.
Additional information regarding FERS is available at http://www.opm.gov/retirement-services/fers-information/