See http://www.customs.gov/xp/cgov/travel/id_visa/esta/

From August 1, 2008, the Electronic System for Travel Authorization (ESTA) is accessible via Internet for citizens and eligible nationals of Visa Waiver Program (VWP) countries to apply for advance authorization to travel to the United States under the VWP.

ESTA is initially available in English only. Other languages will follow. Effective January 12, 2009, all VWP travelers will be required to obtain an electronic travel authorization prior to boarding a carrier to travel by air or sea to the U.S. under the VWP.