Effective January 20, 2010, all foreigners who seek to enter the U.S. under the Visa Waiver Program (VWP) must now complete The Electronic System for Travel Authorization (ESTA) prior to travel. ESTA serves to verify the foreigner’s eligibility to enter the U.S. under the VWP.
If you have a U.S. visa in your passport you do not need to complete the ESTA. If you have no visa and are entering the U.S. using your passport alone under the VWP, then you must complete the ESTA prior to travel.
Which countries participate in the VWP? (March 2016)
The ESTA form has a trick question
Have you ever been denied a U.S. visa or entry into the U.S. or had a U.S. visa canceled?
If you have interviewed for a U.S. visa at a U.S. consulate and the visa was not approved on the day of the interview, then the correct answer is, “Yes”. Immigration services will pull records and verify that your visa application remains pending. If you answer, “No,” and an immigration check reveals you have been interviewed for a U.S. visa that is not yet approved, then there may be unspecified negative consequences for you.
If you are traveling to the U.S. under the VWP, follow this link to complete the ESTA prior to travel.