The Electronic System for Travel Authorization is required for every person traveling to the Unites States by sea or by air. The authorization basically involves an additional vetting process to determine the eligibility of visitors to the country.
It is important to stress that ESTA only provides authorization for travel and does not guarantee entry into the United States. It is still entirely up to a customs and border official at the point of entry to determine whether a traveler should be granted or denied entry.
However, this said, any traveler arriving in the United States without an ESTA visa will not be allowed entry into the country and will immediately be returned by sea or air to their country of origin. The visa does provide for multiple entries over the two year period for which it is valid. When the visa expires, an application for a new visa must be made.
The visa is fully electronic which means that an e-passport (electronic passport) will be required in order for the ESTA approval to be recorded. No document stating that a visa has been approved will be accepted. The passport must also be valid which means that it cannot have expired or expire for the expected period of stay within the United States.
The visa allows for a stay of up to 90 days per trip where after the traveler will be required to leave the country for an equal amount of days +1. If a stay is for 30 days, a visitor may only return to the U.S. after 31 days.
The application process is simple and very fast with approval normally taking about 48 hours. It is however not recommended to leave application to the last minute and rather apply when travel arrangements to the States are being made. Due to the electronic format, all applications must be made online.