GFI or GFCI, same thing - ground fault circuit interrupter - protects you against shock if the ground wire or ground rod has broken or been disconnected
Arc fault interrupter — newer technology — will cut power if there’s an electrical arc between hot and neutral, which won’t trigger a GFI.
http://ecmweb.com/forums/electrical-101/national-electrical-code-nec/new-afci-requirements-2014-nec
http://www.combinationafci.com/resources/doc_ieee_combination_afci.pdf
With alternating current, a nail or screw driven through a piece of Romex can go between two wires and be almost close enough to cause an arc, then add dust or water or movement and you get an intermittent arc (and that only when the circuit happens to be switched on). Eventually the intermittent heat melts something or moves something and you get a hotter arc and a fire.
Arc fault interrupters are slowly being added to the code — first it was kitchen circuits, then I think in 2013 they were added to cover bedroom circuits (and not all of them, and your city and state codes don’t have to follow the US national code of course).
It’s worth reading up on them. Most electricians don’t like installing them because the earlier AFIs were even touchier than the earliy GFIs, and in some circuits you do need both of them increasing the annoyance factor of false positives.
Last work I had the electrician flat out lied to me, said the AFIs weren’t being made any longer, that’s why he wasn’t installing them.
Trust but verify. I’m slowly putting them in for all the circuits, inside and out. So I have to check a little more often for a flipped breaker.
Better than a fire.