The Department of Homeland Security requires all foreign nationals who wish to enter the United States under the Visa Waiver Program (VWP) to obtain permission from the Department's Electronic System for Travel Authorization (ESTA) before embarking on their travel.
ESTA is an automated system used to determine the eligibility of visitors to travel to the United States under the VWP. It requires the same information as the paper I-94W form that VWP travelers furrently fill out immediately prior to their entry to the United States.
The VWP allows citizens of certain countries to visit the United States for business or pleasure for up to 90 days without first obtaining a nonimmigrant visa. Thirty-seven countries currently participate in the VWP -- Andorra, Australia, Austria, Belgium, Brunei, Chile, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Japan, Lichtenstein, Luxemburg, Monaco, The Netherlands, New Zealand, Norway, Portugal, Republic of Malta, San Marino, Singapore, Slovakia, Slovenia, Spain, Sweden, Switzerland, and the United Kingdom.
ESTA applications can be completed at https://esta.cbp.dhs.gov/esta. Travel authorization is valid for two years or until the applicant's passport expires, whichever comes first. A visitor may make multiple entries to the United States during an authorized period. There is no fee for the ESTA application.
ESTA approval does not guarantee admission to the United States. In all cases, a U.S. Customs and Border Protection Officer will still question the traveler upon entry to make a determination as to admissibility. If an individual is denied authorization under ESTA, he or she will be required to apply for a visa at a U.S. consular post abroad. Canadians may enter the United States for business or pleasure purposes without a visa but are not part of VWP.
For more information go to http://travel.state.gov/content/visas/english/visit.html.