The Visa Waiver Program (VWP) allows citizens of participating countries to travel to the United States without a visa for stays of 90 days or less, when they meet all requirements. Travelers must be eligible to use the VWP and have a valid Electronic System for Travel Authorization (ESTA) approval prior to travel. Further information http://travel.state.gov/content/visas/english/visit/visa-waiver-program.html
In order to travel without a visa on the Visa Waiver Program, you must have authorization through the Electronic System for Travel Authorization (ESTA) prior to boarding a U.S. bound air or sea carrier. ESTA is the Department of Homeland Security (DHS), Customs and Border Protection's (CBP) automated web-based system to determine eligibility to travel without a visa to the United States for tourism or business. Visit the ESTA webpage on the CBP website for more detailed information, to apply for ESTA, and pay the fee.
Further information about visas http://travel.state.gov/content/visas/english/visit/visitor.html#overview