Electricity in United States of America / USA is 120 Volts, alternating at 60 cycles per second. If you travel to United States of America / USA with a device that does not accept 120 Volts at 60 Hertz, you will need a voltage converter.
There are three main types of voltage converter. Resistor-network converters will usually be advertised as supporting something like 50-1600 Watts. They are light-weight and support high-wattage electrical appliances like hair dryers and irons. However, they can only be used for short periods of time and are not ideal for digital devices.
Transformers will have a much lower maximum Watt rating, usually 50 or 100. Transformers can often be used continuously and provide better electricity for low wattage appliances like battery chargers, radios, laptop computers, cameras, mp3 players and camcorders. However, they are heavy because they contain large iron rods and lots of copper wire.
Some companies sell combination converters that include both a resistor network and a transformer in the same package. This kind of converter will usually come with a switch that switches between the two modes. If you absolutely need both types of converter, then this is the type to buy.
If your appliances plug has a different shape, you may need a plug adapter.
Depending on how much you plan to travel in the future, it may be worthwhile to get a combination voltage converter and plug adapter.