There are outlets, and then there are outlets that include a ground fault circuit interrupter (GFCI or GFI). GFIs are used to reduce the risk of electric shock when the flow of electricity doesn't follow the correct path, which could result in an electric shock or even death.
The left slot in the outlet is called “neutral,” the right slot is called “hot” and the hole below them is called the “ground.” When appliances work properly, all the electricity used by the appliance flows from hot to neutral. The GFCI monitors the amount of current flowing from hot to neutral, and if it detects an imbalance, it trips the circuit. Once the problem is fixed, you can reset the GFCI manually by pushing the reset button.
There are two types of GFCIs – the circuit breaker type gets installed in the panel, while the receptacle GFI is a special type of electrical outlet. Receptical GFCIs are required anywhere near water – kitchens, bathrooms, unfinished basements, garages, outdoors and anywhere near a sink, e.g. your laundry room.
GFCI outlets also have test buttons so you can test to verify the ground fault interrupter will shut off. GFIs should be tested monthly according to SafeElectricity.org but very few homeowners test them. They're electronic devices that can wear out or get damaged, but you won't know it because the outlet may continue to work. When I bought a house built in 1995, there were GFIs but only one was working. As a result of the home inspection, I asked the seller to have a licensed electrician replace all the GFCIs.