Electrical systems vary in different parts of the world. Plugs used in the United States must be converted for folks traveling abroad needing to charge or use electrical devices such as smartphones and hair dryers.
The type of device needed varies based on the countries being visited and the type of device being powered. Some countries, like Canada, parts of South America, and Taiwan use the same electrical standard as the United States (120V AC at 60Hz), so at most only plug adapters would be needed to fit differently shaped outlets. Also, many devices, like laptops, come with power supplies that can handle a range of voltage (100-240V) so again, at most, only a plug adapter would be needed.
Other countries usually provide power at a higher voltage, so a bulkier transformer for low-power devices (like electronics) or high-power devices (like hair dryers and irons) is needed. Dual transformers that can switch to high-power or low-power, depending on the device type, are also available.