FBI vs CIA. What are the differences?Which one is more powerful?

Excellent answer, and I'd just like to add a little footnote. In many areas of policing, each US state has control over its own legislation, and different states can have differing laws. Where a crime exceeds the boundaries of one state, it becomes the FBIs jurisdiction. In the UK, we have our police forces, Criminal Intelligence (CI0-13), Military Intelligence (MI0-13), NCIS (not to be mistaken for the American Naval Division) and National Domestic Security Service. All 4 of these organisations battle each other for budget funding, and are often antagonistic towards each other, too. In the US, they have the CIA, FBI, Pentagon, and now their new Counter-Terrorism Department. This will not lead to better results, just more backstabbing. If either of you'd like to read a good story about the lack of harmony within the UKs Intelligence branches (though set during the Cold War), read John Le Carre's 'Tinker, Tailor, Soldier, Spy'.

Thank you for the info. I was just wonder what their responsibilities are.

I was never sure what the UK intelligence services did or were responsible for...so thanks for the info. The Joint Counter terrorism force you refer to is supposed (key word) supposed to deconflict information between the other intelligence services and act as a primitive command and control for complex cases; however, this hasn't been working to the degree it was supposed to expect.