If you're travelling from the USA to the UK, or have an American device you want to use in the UK, you'll need a US to UK plug adapter (also called a USA to UK travel adapter), such as our Worldwide Travel Adapter. This converts the flat two-pin American Type A or Type B plug so it fits the UK's three-pin Type G socket.
You'll also need to consider voltage: the UK runs on 230V, while the US uses 120V. Most modern electronics (phones, laptops, camera chargers) are dual-voltage and will handle both automatically; just check the label on your charger for "100–240V". For appliances that are 120V only, you'll need a voltage converter as well as an adapter.