Primary difference between the FBI and the Secret Service?
The FBI is basically a plainclothes police force, that investigates crimes under Federal jurisdiction. The Secret Service’s primary purpose is to protect the integrity of the currency – they are the agency that investigates counterfeiting, bank and wire fraud, and similar economic crimes. As a ‘sideline’ the Secret Service also provides bodyguards for members of the Executive Branch (Or candidates for it) that require them, and investigates threats against them. Yes, Federal Marshals protect judges and courts.