Power converters are used to convert 220/240 VAC to 110 VAC to permit the use of "US" appliances in countries where the commercial power is 220 or 240 VAC. The main consideration is the current draw or power rating of the device to be used with the converter. The converter must, of course, be able to handle the power draw of the device being connected.
A foreign travel adaptor will handle electric shavers, even hair dryers, and appear like a wall adaptor. For appliances requiring more power, a transformer-type is available. The power cord has a US type plug, and comes with an adaptor for use in other countries. They have two output receptacles, and are clearly labeled in output voltage. These converters will convert either way, 110 to 220, or 220 to 110, and are available in a number of wattage ratings from 100 to 2000 watts.
In most other countries, the commercial AC power frequency is 50 Hz, where ours is 60 Hz. This can cause a problem with some electronic devices that require the 60Hz AC power, such as alarm clocks. Video components such as TV sets, VCRs, and DVD players, use a different video system than is used in the US , so they generally won't work anyway. Some LCD-type displays can be switched between the US system, NTSC, and the PAL system used in Europe and other locales. These will of course work with the converters, but are the exception. Appliances such as electric shavers, hair dryers, and most musical instruments, do not require a specific frequency, and it is these products that the converters were intended to be used with. Many digital musical instruments such as keyboards will work with 50 or 60 Hz power as they are clocked internally. Some consumer electronics such as cassette tape recorders may or may not work properly on converted power.