ESTA (Electronic System for Travel Authorization) is a not so well-known automated system that was built to determine eligibility for people traveling to the US under VWP (Visa Waiver Program). The system is straightforward but since not much is known about it, most people make assumptions that are incorrect. One of the assumptions is that everyone needs an ESTA approval. In reality, you only need this in some specific situations. 

When VWP Applies

Whenever you are a citizen of a country that is covered by the Visa Waiver Program and you enter the US by land (ie. by crossing from Canada), you do not need ESTA, just as you do not need one when you are a Canadian citizen. However, if you travel by sea or air, ESTA application is always necessary for VWP countries. When you are not a VWP country citizen, a visa is needed. Because of this, ESTA will not apply. 

All lawful US residents with VWP country citizenship do not need ESTA. However, even if you will be just in transit when you land in US in this case and you are just a VWP country citizen, ESTA is needed. 

Those that have citizenships in non-VWP and VWP countries would need ESTA when traveling with the passport issues by the VWP country. Visa is necessary whenever you travel with the passport issued by the non-VWP country. 

If you are a US citizen you do not need to have ESTA when you travel and you need to use the US issued passport when entering or exiting the country. This also applies to those that have double citizenship. 

When Visas Are Involved

When you have a valid visa you can use it in order to travel to the US but only for the purpose for which that visa was officially issued. There is no reason to apply for ESTA if the visa is valid. 

Other Facts Of Interest About ESTA 

One thing you should know is that there is a fee that you have to pay for ESTA verifications. This fee is two-fold:
• The processing charge – You are basically charged $4 for application processing. 
• 2009 Travel Promotion Act – This requires an extra $10 you have to pay as an ESTA applicant approved for US travel under VWP. When electronic traveling authorization is denied, the processing fee is the only charge. 

When you use the services of a third party to handle your ESTA application, the company will charge an extra fee. This is not an official fee you are required to pay. 
Remember that you have to print a document copy for personal records. Printouts are not required when you enter the US since officers have access to your information in electronic format. However, there are some airlines that will require printouts as check-in happens. Be sure you check with the airline to see if this is necessary or not.

On the whole, the ESTA process is one that is simple but that can turn out to be complicated for some. Whenever you feel you need help, look for someone to offer it.