What is the ESTA?
ESTA stands for Electronic System for Travel Authorisation, it is not a visa, but is part of a post 9/11 system to increase security for travellers entering the US from countries included in the Visa Waiver Programme, such as the UK.
Who needs it?
Any UK national travelling to the US as a tourist or on business for 90 days or less must have an ESTA or they will not even be able to board a flight. Even if you are just passing through America in transit, you are still required to have a valid ESTA.