A 120-Volt Outlet
Typical outlets in U.S. homes and businesses have a rating of 120 volts. Specialty outlets for larger items such as a refrigerator, electric clothes dryer or 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.
While there is a major difference in electrical outlets between U.S. and Europe, the standard voltage for a wall outlet in Europe is 220 volts. So it is better to carry a power converter when travelling. If the input voltage 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, then your 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.