The FBI, or Federal Bureau of Investigation, is an important faction of criminal investigation within the United States. They handle federal crimes, and head investigations that cross over state lines that local police to not have the jurisdiction to conduct.
Because they are a centralised government agency they are able to recruit candidates of the highest calibre, and spend a great deal of time and money training their recruits to become highly specialised agents.
They are instrumental in the the prevention of terrorism and attacks threatening national security. The introduction of the USA PATRIOT Act gave them even greater powers in this area.
The FBI often works in conjunction with other Federal agencies, including the Coast Guard and Customs and Border Protection in airport security, and the National Transportation Safety Board in investigating plane crashes. They are also an key part of the Joint Terrorism Task Force.