Electric voltage provided to home wall outlets differs by country. In the United States, the voltage is at 110 volts (V). The rest of the world uses 220V or 210V. For U.S. appliances to work outside of the U.S., the voltage will need to be changed.
A step down transformer is used to reduce the electrical voltage in the line to one that will not damage the appliance. Step down transformers reduce the voltage by multiples, so 220V is converted to 110V. The transformer will not lose any power in the step down and is used for alternating current (AC)-powered appliances. Without the step down transformer, the appliance can be damaged by the high voltage. Plug the transformer into the 220V wall socket and then plug the appliance into the transformer.
For those that have electrical repair experience one of the wires used in the 220V circuit can be disconnected. A 220V circuit contains two, 110V wires. Simply disconnect one of the wires for the plug that uses a 100V appliance. This can get tricky as the wire gauge may be wrong and instead of a double pole breaker, a single pole breaker will be needed. Check with the local electrical requirements.
A plug adaptor will not converter the voltage for the appliance. The plug adaptor makes it possible to plug a 110V into a 220V socket but will not convert the voltage. A step down transformer will have the correct plug for the 220V wall socket. However if the wiring in the circuit is altered, then a new wall socket will be needed or a plug adaptor for the appliance.