ESTA or Electronic System for Travel Authorization is an automated system used by the Government of the United States of America to determine the eligibility status of citizens from selected countries to travel to the U.S. under the Visa Waiver Program, without having to previously apply for a USA visa. Starting on January 12th, 2009, all individuals planning to enter the country under this program, must be holders of a valid ESTA visa waiver for the USA.
The processing time for an ESTA visa waiver is up to 24 hours, which means candidates can make their online application prior to boarding the plane.
Take note an ESTA America is not a visa, therefore valid US visa holders do not need to apply for the travel authorization, as an ESTA and a visa cannot be granted simultaneously. ESTA holders will not be required to fill in the Green I-94 Card upon their arrival in the US. However travellers will need to fill in a blue customs declaration, whether they are ESTA holders or not.
ESTA USA has a validity period of 2 consecutive years. During this time successful applicants may travel multiple times to the US for short periods of time ( up to 90 day per stay). Take note that an overstay (over 90 days) will render your ESTA ineligible for any future entries and/or applications.
It is recommended to travel to the US in reasonable intervals of time and not to abuse the rights given to US ESTA holders. Traveling back and forth for long periods of time might cause the border officials to believe you are intending to live there.