ESTA (Electronic System for Travel Authorization) is a legal permit, authorizing successful applicants to enter the United States under the Visa Waiver Program (VWP). Applicants from 39 countries, including the UK, Australia, and Ireland, can take advantage of the program.
ESTA is an eligibility evaluation, which aims at pre-screening travelers to the U.S. Once their eligibility and safety are confirmed successful candidates are authorised to travel to America. However, the final decision about whether or not they will be allowed in the country will be taken into further consideration once at the border control of the United States.
An officer will review the applicant’s eligibility status and make the final decision to whether or not you may cross the border. Note that the majority of successful applicants do not experience problems entering the country.