The Electronic System for Travel Authorization (ESTA) is an online application system used by travelers from Visa Waiver Program countries to enter the United States for tourism or business purposes. It is designed to enhance security and streamline the entry process. Travelers must apply for ESTA at least 72 hours before their flight. Once approved, ESTA allows multiple entries into the U.S. for up to two years or until the traveler's passport expires, whichever comes first. It is important to note that ESTA does not guarantee entry; U.S. Customs and Border Protection officials make the final decision upon arrival.