Electricity in Bhutan is 230 Volts, alternating at 50 cycles per second. If you travel to Bhutan with a device that does not accept 230 Volts at 50 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 need both types of converter, then this is the type to buy.