In the U.S., the historical standard for household equipment is to accept a minimum of 110 volts; in Europe, this number is 220 volts. Occasionally, this number is misread as watts, which is not the same type of unit.
In the U.S., household electricity is supposed to arrive at 120 volts. However, poor household wiring, long extension cords and power company issues often cause it to arrive with slightly less or more, hence the original 110-volt minimum. Appliances tolerate minor fluctuations, but the range for which a U.S. appliance is built is well under the minimum accepted by a European appliance, and attempting to plug a 120-volt appliance into a 240-volt outlet invites overheating and electrical fires. To prevent this kind of accident, the plugs on U.S. and European equipment are of different shapes. Some equipment marketed to international travelers accepts both voltages, and many computer power supply units have a switch to toggle between the two.
Referring to 110 or 220 watts instead of volts represents a common misunderstanding of electricity. A watt is a unit of power, whereas a volt is a measurement of electrical pressure, as it is colloquially known. Watts are equal to volts times the amount of electrical flow (current) in amperes.