The Electronic System for Travel Authorization, typically abbreviated as ESTA, is the automated system used by the U.S. to determine the eligibility of tourists and visitors to travel to the country. It falls under the VWP (Visa Waiver Program) and checks whether such travel posts any security or law enforcement risk.
With ESTA, therefore, you will be allowed to travel to the United States under this Visa Waiver Program. However, you ought to note that it is not a visa. Additionally, it doesn’t meet the legal requirements needed to serve in place of a regular U.S. visa where such a visa is required.
In general, your ESTA authorization would typically be valid for several trips over a 2 year period from the date you received the approval, or until your passport expires (whichever date comes first). Therefore, so long as you have ESTA authorization to travel into the United States, you won’t have to reapply during the period it is valid.
In the same way, if the ESTA expires when you are within the US, your departure won’t be affected.
NOTE: You must PRINT copies of the document for your own records. These printouts won’t be required when you arrive because the officers will be able to access the information electronically. However, some airlines do require that you show the printout while checking in. This means that you need to check with the airline you will be traveling on to find out whether an ESTA printout is required.
ESTA approval does not mean that you can stay in the country for the 2 years you have received. The authorization only allows you to travel into the country under the terms of the VWP (Visa Waiver Program). This program only allows foreign residents to stay in the United States for 90 days or less. Consequently, if you wish to stay longer than the allowed 90 days, you need to obtain the right visa from the nearest U.S. Consulate or Embassy.
In case you change your country of citizenship, gender, or name, or if you get a new passport, you must approve for fresh travel authorization. This is also a requirement if your answer to any VWP eligibility question changes. New applications come with associated fees of $14 for every submission.
NOTE: While it is highly recommended that you apply for ESTA approval a minimum of 72 hours before you are scheduled to travel, it is still possible to apply any time before you board your flight. In most cases, you will receive a response seconds after you submit your application.
Overall, ESTA approval does not guarantee that you will be allowed to enter the United States. However, it will make the process at the port of entry that much easier and faster.