The AFCI (Arc Fault Circuit Interrupter) breaker will shut off a circuit in a fraction of a second if arcing develops. The current inside of an arc is not always high enough to trip a regular breaker. You may have noticed a cut or worn piece of a cord or a loose connection in a junction box or receptacle arcing and burnt without tripping the regular breaker. As you can guess this is a major cause of fires in a dwelling.
Starting January 1st 2002 the National Electrical Code, Section 210-12, requires that all branch circuits supplying 125V, single phase, 15 and 20 ampere outlets installed in dwelling unit bedrooms be protected by an arc-fault Circuit interrupter. Eventually they will be in more areas but the NEC selected to require them on bedroom circuits first because a CPSC study showed many home fire deaths were related to bedroom circuits.
There is a difference between AFCIs and GFCIs. AFCIs are intended to reduce the likelihood of fire caused by electrical arcing faults; whereas, GFCIs are personnel protection intended to reduce the likelihood of electric shock hazard. Don't misunderstand, GFCIs are still needed and save a lot of lives.