The Visa Waiver Program (VWP) is administered by the Department of Homeland Security (DHS) and enables eligible citizens or nationals of designated countries to travel to the United States for tourism or business for stays of 90 days or less without obtaining a visa. Instead, citizens or nationals of the designated countries must request an authorization to travel under the Electronic System for Travel Authorization (ESTA).
ESTA is an automated system that assists in determining eligibility to travel to the United States and whether such travel poses any law enforcement or security risk. Upon completion of an ESTA application, a traveler is notified of his or her eligibility to travel to the United States under the VWP. If a traveler is refused under the ESTA program, he/she must apply for a visa at the nearest U.S. Embassy or Consulate.
Further information and instructions can be found at: https://esta.cbp.dhs.gov/esta/