In most of the world, household outlet voltage is 220 volts. In the United States and neighboring countries, however, household outlets run at 110 or 120 volts. This can pose a serious problem for travelers. Connecting a 220 volt appliance to a 110 volt outlet can damage or destroy the appliance. Fortunately, voltage adapters, which completely fix the problem, are cheap and easily available.
TL;DR (Too Long; Didn't Read)
Always make sure to consult travel guides to ensure you have the correct adapters. Some companies sell dual-voltage appliances, designed for travelers, that can be used with different voltage outlets.
International Plug Shapes
Though several countries operate on a 220-volt standard, plug styles vary. This is an important consideration, as many are mutually incompatible: you cannot use a Japanese-style plug (flat blades) in an Italian-style outlet (round pins), for example. So it pays to know the exact plug type you'll be needing for your appliances. In some instances, the appliance manufacturers have designed their equipment for international use. Some travel appliances come with a universal end plug that accepts different slide-in adapters, provided with the appliance for your convenience.
Converting an Appliance from 220 to 110
Buy a 110 volt to 220 volt adapter. While this may sound backward, remember that the wall voltage is 110 volts and your appliance needs 220 volts to run, so you need a device that will take 110 volts and step it up to 220 volts (in short, 110 mains to adapter to 220 device). These are available in most electronics stores and usually cost less than $20. Many of them will also say explicitly on their packaging that they are designed to let you use European appliances in the United States.
Connect the voltage adapter to the 110 volt wall outlet. Depending on what part of the world you are in, you may also need an outlet adapter to connect the voltage adapter. Most travel guides have information on the types of outlets that are used in different countries. Before traveling, consult one of these to make sure you will be able to plug your voltage adapter into the wall outlets. Outlet adapters are also inexpensive and available in most places that sell electronics or travel supplies.
Connect your 220 volt appliance to the outlet on the 110 volt to 220 volt voltage adapter. Verify that the outlet shape on your voltage adapter matches the outlet shape your appliance uses. If it does not, you can attach another outlet adapter to your voltage adapter to make it match with your appliance.
Use your appliance normally. If your voltage adapter is functioning properly, you should notice no change in the function of your appliance.