The little buttons on electrical outlets marked as "test" and "reset" are specially designed to better protect people from electric shock than ordinary outlets. GFCI's or GFI's as they are commonly called, have been used in homes since the 1970's, although most notably in the last ten years.
GFCI's are designed to shut power off to the outlet if there is a very small variance, interruption, or "leak" of any electricity, which ordinary outlets don't detect. Normal outlets turn off by a fuse or breaker if more than 15 amps flows through the breaker. Fuses or breakers prevent the wires from over heating, thus preventing fires. But fuses and breakers do not protect people from electrocution. Surprisingly enough, people can be killed by just 1 amp of electrical current. GFCI's turn off power if a variation as small as .005 amps occurs.
How do they work?
A GFCI detects a small "leak" or variance in electrical current by comparing how much electricity comes back through the neutral (white) wire to how much was sent in the hot (black) wire. If just .005 variance in amperage between the two wires is detected, the GCFI "trips" and will not allow electricity to pass through it. Appliances that are malfunctioning, power surges and moisture in the outlet can cause these variances.
People can be an excellent grounding source for a "leaking" outlet. The unsuspecting person may get a shock from a normal outlet, but is protected from a properly functioning GFCI outlet because the GFCI trips at the slightest fluctuation in electricity.
Where are they used?
GFCI's are now required by code for outdoor outlets and all areas within six feet of a water source inside the home, which includes bathrooms, laundry rooms, garages, basements, and the kitchen.
Do all GFCI's have buttons?
No. Some outlets are GFCI protected at the electric panel box with a special GFCI breaker that performs the same function as a GFCI outlet for all outlets running to that breaker. Any outlet wired in series downstream of a GFCI outlet or GFCI breaker is protected. So, some normal looking outlets may be GFCI protected. Most hardware stores sell GFCI testers that will trip a GFCI when used.
Can older houses have GFCI's?
Yes. GFCI's can be added to any electrical system. Home Check America inspectors recommend they be installed whenever they are absent in a home. Although they do not replace the grounding system of the home, some codes do allow them in place of grounding in some cases. They are more expensive than regular outlets ($10-$15 vs. 50-90 cents), but are an inexpensive protection from electric shocks.