A GFCI (ground-fault circuit interrupter) outlet protects against electric shock by cutting power in a fraction of a second when it senses current leaking to ground — for example through a person. They're required anywhere water and electricity meet.
Where they're required
Code requires GFCI protection in kitchens (counter receptacles), bathrooms, outdoors, garages, unfinished basements, and near sinks and pools. Older homes may not have them, and adding GFCI protection is an inexpensive, worthwhile upgrade.
Test and reset
GFCI outlets have Test and Reset buttons. Press Test monthly — the Reset button should pop and power should cut. Press Reset to restore it. If pressing Test doesn't trip the outlet, it's faulty and should be replaced.
Tripping GFCIs
A GFCI that trips repeatedly may be protecting you from a genuine ground fault in an appliance or in damp wiring. Unplug devices to isolate the cause. If it trips with nothing plugged in, have an electrician check the circuit.