Electronic System for Travel Authorization (ESTA)
Since January 12, 2009, all nationals or citizens of Visa Waiver Program (VWP) countries who plan to travel to the United States for temporary business or pleasure have been required to obtain prior travel authorization through ESTA prior to boarding a carrier to travel by air or sea to the U.S. under the VWP.
In order to avoid unnecessary inconvenience and delays, all Visa Waiver travellers to the U.S. are reminded that they must obtain prior travel authorization by providing the necessary information in the ESTA form available at the link below.
Please find the details at the Department of State: http://travel.state.gov/visa/temp/without/without_1990.html
This does not apply to U.S. citizens traveling abroad.