An Arc-fault Circuit Interrupter (AFCI) or Arc-fault Detection Device (AFDD) is a circuit protective device designed to prevent fires caused by arcing. The device is intended to detect the characteristic current profile caused by electric arcing and disconnect the affected circuit. Ideally they will detect line to earth as well as line to neutral (i.e. in circuit) arcs. (Note that an RCD will detect Line to earth current flow, but not L-N arc currents since these aren't a current imbalance).
AFCI devices are popular in some countries (notably the US), but have not so far been commonly found in domestic situations in the UK. It is believed that the forthcoming UK Wiring Regulations (18th edition) may include protection by AFCI/AFDD (although it is not yet known in what circumstances it will be recommended or even mandated).
 Domestic fires caused by arc faults are far more common in the US due to the use of lower voltage supplies (and hence higher circuit currents), use of aluminium cables, and also the relatively poor quality of their wiring accessories).