When transiting through an American airport, you are required to hold an ESTA. This requirement applies regardless of whether or not the traveler leaves the airport area during their connection.
Applying for an ESTA is quick and easy
If you have connection through an American airport, applying for an ESTA is both quick and easy. You can apply and pay for your ESTA authorization online. After your application has been processed, you will receive a response at your email address.
You can use one ESTA authorization for all your transfers
Since an ESTA authorization is valid for an unlimited number of trips during its 2 years of validity, you do not need to re-apply for a new ESTA for each transfer you make in the US. As long as your ESTA and passport remains valid, you can freely travel to and through American airport. You also do not need to worry about updating the itinerary information originally provided in the application form. As long as the personal information (i.e. your name and passport number) remains up to date, you are good to go.
An ESTA obtained for transit can also be used for entering the US
Even if an ESTA is originally obtained for the purpose of transiting to another country, it is also valid for entering the US for the purposes of tourism/leisure, business and visiting family and friends. You do not need to update your application, and it will have the usual 90 day stay allowance.
You can leave the airport during your connection
Since an ESTA both authorizes transit and entry, you are free to leave the airport area during your connection if time permits. Just keep in mind that you may have to go through immigration, which can often be slower than in e.g. European airports. So if you just have a short connection, it may be more advisable to stay inside the airport.
Is an ESTA required for both transfers and transits?
Regardless of whether you are just making a transfer (i.e. simply changing planes) or a transit (i.e. leaving the airport overnight), you need an ESTA regardless. Simply put, anyone boarding a plane bound for an American airport must have either an ESTA or a visa, provided that they are not American or Canadian citizens.
Always verify your ESTA status before flying to an American airport
Regardless of whether you are just in the US for less than an hour changing planes, going there for your months-long dream vacation, or anything in between, you must hold a valid ESTA authorization before you will be allowed to board. In order for your ESTA to be valid, the passport with which you originally applied must also remain valid. If you received a new passport in the meantime, you also have to apply for another ESTA.