You didn't hear it from me but...
The Security Service covers counter-espionage, counter-terrorism etc. (threats against the UK), whereas SIS gathers intelligence about foreign powers (in other words MI6 spies on people, MI5 stops people spying on us, or blowing us up), as does the CIA.

In the US, the FBI covers counter-espionage and counter-terrorism among other things.
(Vice Admiral Sir Charles Cockbrush KCB MBE DSO, who is finding it increasingly difficult to log into b3ta.com wants to duck your aunt and kick your puppy, Mon 15 Feb 2010, 16:44, 2 replies)

Aah I see, it's all becoming clear now. MI6 are James Bonds and MI5 are counters (so a little like MFI then, presuming they sell counter tops for kitchens? Perhaps Nicola's mum was right all along). Mind you now that I know I'll have to kill myself in case I tell anyone else. Oh well, helps with the population problem I guess :O)
(purplelizzie, Mon 15 Feb 2010, 16:59, closed)

I've done counter-espionage
when buying a porn mag in a newsagents.
( sandettie light vessel automatic New Twitter - @bollocksreally, Mon 15 Feb 2010, 18:14, closed)