People who can look like 7 other people in 5 minutes and have 18 different passports with 35 different names.
Work for the part of government who thinks they are the smart ass and center of the world. People usually have a love-hate relationship with them, in the sense that you get amazed by the jobs they do but also hate their snobby know-it-all attitude. In movies they are the people who flash a wallet and roar "FBI!" as they chase after terrorists with legit looking guns.
Favorite phrase is "that information is classified." This phrase is also the reason why people think they are snobby know-it-alls.
Person 1: Who wear sunglasses on a rainy day looking like they are going to the concert in that suit?
Person 2: Some kind of FBI Agent...
Loli: Onii-chan, what is thiswhite liquid in my pee hole?
FBI: *thump* *thump* *thump* FBI OPEN UP! *doors and windows blow up and rifles get fired rapidly*
Basically, the FBI warning screen that usually appears before the beginning of a show. The FBI warning of doom is also presented before a movie on videotape starts. They're also in DVD movies, sometimes showing up at the end of the movie, or sometimes at the beginning.
A member of the FBI that looks on you through your phone or device. He is typically depicted as being benevolent in nature, helping out many people in their lives. Despite this he is still watching you.