A ground-fault circuit interrupter, also called a GFI or GFCI, shuts off a circuit instantly if it senses a hazardous short. The receptacle type looks like a regular receptacle except it has a reset button in the middle.
In new construction, these are usually required in bathrooms, kitchens and outdoors. Often a GFCI is mounted as the first receptacle along a circuit (between the electrical panel and other receptacles on the same circuit), automatically protecting the others on the same line.
--Don Vandervort