Arc-fault Circuit Interrupters (AFCI)
Arc-fault circuit interrupters are based on newer technology. (These are not GFCI (Ground Fault Circuit Interrupters. Click here for information on GFCIs.) The purpose of AFCIs is to to detect a small spark that could cause a fire and shut off the breaker. The spark could be due to a break in a wire, a short circuit, bad connections, etc. The problem is that when you flip a switch or plug in or unplug an appliance that is turned on, you get a small spark. No one wants to have to constantly be resetting breakers in normal daily living. The design has been improving so that these very sensitive devices do not cause nuisance trips; time will tell.
Here's a link for more technical data from the US Fire Administration web site.