Individuals from certain countries are able to travel to the United States without a visa. A list of Visa Waiver Countries and more information on the Visa Waiver Program can be found here:
The Electronic System for Travel Authorization (ESTA) is an online system that is part of the Visa Waiver Program (VWP). ESTA is mandatory for all nationals or citizens of VWP countries who plan to travel to the United States for temporary business or pleasure. Under the VWP, all such travelers must receive an electronic travel authorization prior to boarding a U.S.-bound airplane or cruise ship.
ESTA will determine, almost immediately in many cases, if an individual is eligible for VWP travel, and if such travel poses any law enforcement or security risks. ESTA applications may be submitted at any time prior to travel, and once approved, will be valid for up to two years or until the applicant’s passport expires, whichever comes first. Authorizations will also be valid for multiple entries into the United States. In order to facilitate the authorization process, the Department of Homeland Security recommends that ESTA applications be submitted as soon as an applicant begins planning U.S.-bound travel, and not less than 72 hours prior to travel.