Arc Fault Breakers and their uses.
Electrical arcs are breakdowns of gas that produce a constant plasma discharge. Basically, electricity is flowing between two objects, usually caused by an electrical short. When a circuit has enough current to maintain an electrical arc outside of a normal switching device, the arc can seriously damage equipment. Conductors can be melted, insulation destroyed, and, worst of all, fires can be started. The arcs form new chemical compounds from the surrounding air, including oxides of nitrogen and ozone, which you can smell; these chemicals will eat nearby metal surfaces.
Arc-Fault breakers, are circuit breakers designed to prevent fires by detecting arc faults and disengaging the power; the flow of electricity will be interrupted and the arc will be killed. It has been calculated that over 40,000 fires are attributed to faulty home wiring, resulting in nearly 400 deaths. Arc fault breakers check the current and are programmed to differentiate between intended and unintended arcing conditions. Arc fault breakers are not perfect and can read false positives, disengaging the power when no such electrical arcs exist.
For more information on determining when to use an arc fault breaker, check out this article from wikiHow.