The ESTA is a travel authorisation that is issued by the US government to people who are eligible for visa-free travel to the country. The ESTA is issued electronically, and pre-verifies that people are eligible to travel to the United States for purposes that do not require a traditional visa.
People from the UK who wish to apply for an ESTA are required to have an electronic passport. Most people are likely to hold one of those passports now, as they have been available for a long time, however it is possible that someone whose passport is close to expiring may still hold one of the older, non e-passport designs.
E-passports feature an electronic chip which contains a record of the information that is also printed on the data page. These passports have security features that will prevent an unauthorised party from reading the data on the chip. E-passports can help to prevent identity theft, and make it easier and quicker for people to pass through immigration by having the passport scanned electronically.
Someone who holds an older passport will need to apply for one of the new designs before they apply for an USA ESTA. It is possible to apply for an ESTA even if you do not have travel plans in the immediate future. It is a good idea to do this if you expect that you will travel soon, because the ESTA will remain valid for more than one trip.
In most cases, the ESTA is authorised within minutes. There may be some things that would impact the approval process and delay the application even for someone who is eligible to receive an ESTA, however. This means that it is a good idea to apply early, just in case there are questions about your application or there is an issue that slows down the approval process.
If you are traveling to work, compete as a professional athlete or do journalism, or to stay for longer than the period permitted under the Visa Waiver Program then you will need a visa. If you are not sure if you would need a Visa, then you should contact the embassy in your country to confirm the requirements for your trip. They will be able to tell you what you need, and help you with the application process, or put your mind at rest that a visa is not needed for your stay.