ESTA is an automated system that determines the eligibility of visitors to travel to the United States under the Visa Waiver Program (VWP). Authorisation via ESTA does not determine whether a traveller is admissible to the United States. U.S.Customs and Border Protection officers determine admissibility upon travellers' arrival. The ESTA application collects biographic information and answers to VWP elibigibility questions. ESTA applications may be submitted at any time prior to travel, though it is recommended that travellers apply as soon as they begin preparing travel plans or prior to purchasing airline tickets.
Once granted, an ESTA authority is valid for 2 years (or less if a passport expires or for any reason has to be changed in that period). Before each new visit to the U.S., the passenger can update travel plans and contact details via the ESTA website (though this is not currently mandatory). If any traveller's personal details change on the passport, a new ESTA must be completed. If the traveller already holds a valid visa for the U.S. an ESTA is not required. The current charge is $14 per application.
Important Notice - As of 1 April 2016, you must have an e-passport to use the VWP. An e-passport is an enhanced secure pasport with an embedded electronic chip. You can readily identify an e-Passport, becuase it has a unique international symbol on the cover.