It is the investigative arm of the US Department of Justice, a cabinet branch headed by the Attorney General. Its primary roles are preventing terrorism within the country, enforcing Federal laws (including those on civil rights), and providing support for other police agencies (including local, state, and international law enforcement services).
The head of the FBI is its Director, who has a Deputy Director and several Assistant Directors in charge of various offices and projects. It has its Headquarters in Washington, DC, and field offices around the country. The forensic FBI Laboratory ("crime lab") and the FBI Academy are located at Marine Corps Base Quantico, in Quantico, Virginia.