Typical outlets in U.S. homes and businesses have a rating of 120 volts. Specialty outlets for larger items such as a stove, electric clothes dryer or a large-BTU air conditioner carry more voltage, but the vast majority of the wall outlets have two-prong outlets with a third hole for grounding.
Older standard outlets in U.S. homes only feature two prongs without a third hole for grounding. Replacing and in some cases rewiring these outlets are necessary to use devices with a three-prong plug in those sockets.
People from the United States who travel to Europe should prepare for a major difference in electrical outlets. The standard voltage for a wall outlet in Europe is 220 volts, and just plugging a U.S.-manufactured hair dryer into a European plug can change that hair dryer into a clump of plastic while the electricity shorts out.
When traveling, carrying a power converter for those appliances that require it is useful. Looking on the back of electrical appliances tells the owner whether he needs a power converter or not. If the input on the label is only 120 volts, then a converter is necessary. If the input voltage has a range that goes as high as 220 or 240 volts, that device is ready to work in Europe as well, so all the owner needs is a plug adapter for the different input on a European outlet.