The United Kingdom and United States use two different standards of electrical current for appliances. The UK uses 240 V, whereas the US operates on 110 V. Naturally, this can make using an appliance bought in the UK difficult in the US. Apart from differently-shaped plug adaptors, the difference in current can cause the appliance to be non-functional, or may even cause permanent damage. Some appliances can be used on both sides of the ocean with the proper equipment.
Examine your item and the power supply. There will be a description of the current, expressed in voltage, and possibly a hertz (Hz) measurement. These figures will determine what equipment you need to run your appliance. If an item has an internal power switch on either the unit or within the power adaptor, it may be expressed in a variable voltage number. Items that require a step-down transformer will say "240 V" only, whereas items that have an internal power switch will say "110 V - 240 V," or similar.
Look for a hertz frequency. This is common on items with motors, such as kitchen appliances. The US uses a 60 Hz current, whereas the UK and Europe use 50 Hz currents. If an item has a hertz frequency that is not expressed as being variable (for example "50/60 Hz"), then the item cannot be used on US power, as it will be unable to function properly.
For items with an internal power switch, plug the power cord into a plug adaptor. This adaptor allows the item's plug to be accommodated by the wall socket and does not change the current or frequency. Plug the adaptor into the wall socket and the item should power on as normal.
If the item does not say "110 V - 240 V," you will need a step-down converter, also called a transformer. Plug the item into the converter and plug the converter into the wall socket. The item should run as normal.
If an item does not say 50/60 Hz, it will not work in the US. Items such as televisions may work, but they may be incompatible with peripherals such as DVD players or gaming systems. When in doubt, refer to the manufacturer. If an item does not work properly, double check the voltage and hertz frequency.
Do not plug items that require a step-down converter into a plug adaptor.