Entering the USA?
A citizen of a foreign country, wishing to enter the USA, generally must first obtain a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence.
The type of visa you must have is defined by immigration law, and relates to the purpose of your travel. Having a U.S. visa allows you to travel to a port-of-entry (airport, for example) and request permission of the Department of Homeland Security, Customs Border Protection immigration officer to enter the U.S. A visa does not guarantee entry into the United States. For more information: visitors visas.
VISA WAIVER PROGRAM
Foreign citizens traveling for visitor visa purposes, from certain eligible countries may also be able to visit the U.S. without a visa, through the Visa Waiver Program (VWP) if they meet requirements.
All travelers seeking admission to the US under the VWP must present a machine readable passport.
What is the Visa Waiver Program?
The Visa Waiver Program (VWP) is administered by the Department of Homeland Security (DHS) and enables eligible citizens or nationals of certain countries to travel to the United States for tourism or business for stays of 90 days or less without obtaining a visa.
What Countries Are in the VWP?
Andorra, Australia, Austria, Belgium, Brunei, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Iceland, Ireland, Italy, Japan, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Monaco, the Netherlands, New Zealand, Norway, Portugal, San Marino, Singapore, Slovakia, Slovenia, South Korea, Spain, Sweden, Switzerland and United Kingdom.
The Secretary of Homeland Security, in consultation with the Secretary of State, may add countries to the program or remove them from the program at any time.
Source of information: US Department of State.