The NSA and CIA are both supposed to be for foreign intelligence only. Only the FBI is supposed to directly conduct domestic operations, though frequently they

The NSA and CIA are both supposed to be for foreign intelligence only. Only the FBI is supposed to directly conduct domestic operations, though frequently they do it with the assistance of the NSA. And the NSA has interpreted "foreign" as anything involving at least one foreign party. The difference between the CIA and NSA is that the CIA primarily does HUMINT (human intelligence), while the NSA does SIGINT (signal intelligence). HUMINT includes spies directly living in the target countries, mingling with the people they are spying on. SIGINT involves intercepting communications and decrypting them. There is some crossover; in particular, HUMINT can frequently be used to place bugs, keyloggers, rootkits, and the like which can be used for SIGINT, and SIGINT can fill in information that HUMINT is not privy to.