The standard voltage system used in US is 110 volts but every other nation has their own voltage system. For example: Japan uses 100 volt while Philippines, Korea and China uses 220 volt as their standard voltage system. May of the hotels provide adapters to their customers which can make life easier though one must be educated about how to use the devices.
Adapters, Converters and Transformers, their function might be the same but usage are different. While adapters can be used easily but converters can be used with devices that have simple mechanical motors. The converter steps down 220 voltage to 110 volts thus enabling devices to operate on that voltage supply platform. If one uses the device without converters, it will simply burn it.
Buying 110 voltage converters is quite easy. Most of the sellers know your requirement and advise accordingly. You must tell them the name of country you are traveling so they will be able to analyze the requirement and provide you the solution.