The differences between the Federal Bureau of Investigation (FBI) and the Central Intelligence Agency (CIA) are quite broad in most regards...
The FBI is basically a police orgnization. Their job is to fight major criminal activity. Things like bank robbery, money laundering, major fraud, kidanpping, copyright infringement, etc... these and other things that the FBI covers.
The FBI also has something of an intelligence function. This is where things get a wee bit murky. The FBI is responsible for counter-intelligence here in the US. This means spy-catching. They are also responsible for counterterrorist operations. Here is where they get entangled with CIA at times.
The CIA is the senior intelligence agency in our government... but not the only one. The CIA is responsible for the collection of foreign intelligence abroad, and is genereally forbidden from intelligence activities within our own borders.
The FBI and CIA clash from time to time when their operations overlap. There is a lot more to it, obviously... but there is the bare bones of it.
There... that should be as clear as mid, eh?
Bill