ESTA stands for Electronic System for Travel Authorization. It is an online application system introduced by the US government to determine the eligibility of a visitor to travel to the US under the Visa Waiver Program.
Visa Waiver Program, started on 12th January, 2009; authorises citizens of specific countries to travel to the US for tourism or business for upto 90 days, without obtaining a ESTA Visa US. The countries selected under this program are high-economic countries generally termed as developed countries.
ESTA is not a visa, it is merely an authorization given to a traveller to board without a visa while travelling through sea or air. It does not provide admissibility rights to the traveller; the same is determined by the U.S. Customs and Border Protection (CBP) officers upon arrival.
Apply ESTA USA is valid for 2 years or until the expiry of the passport whichever is earlier. It allows a short stay of 90 days or less to the visitor. ESTA USA Travel can be used for multiple entries within the specified tenure but the combined stay should not exceed 90 days. However, a traveller should obtain a new ESTA in case he/she is issued a new passport; or there is a change in his/her biographical information, e.g. name, gender, country, citizenship etc; or if any of the answers to ESTA USA Application Online eligibility questions changes.