Citizens from countries participating in the US Visa Waiver Program who wish to travel by air to the United States must obtain prior approval through the Electronic System for Travel Authorization (ESTA). ESTA is used by the U.S. government to screen eligible residents of Visa Waiver Program (VWP) countries before they board a plane to the U.S to determine whether or not they pose a law enforcement or security risk. VWP passengers are encouraged to apply for an ESTA when beginning to plan travel to the United States, using the convenient ESTA online form External site which may not meet accessibility guidelines. Passengers will be required to present proof of ESTA registration at time of check in and those who have not obtained prior approval via ESTA before departure will be denied boarding. 

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Four giant airline computer systems in the United States handle nearly all the airline reservations in the country. (They're known as CRSs, for computer reservations systems, or more often now GDS for global distribution systems.) Although each airline has a ``home'' CRS, the systems are all interlinked so that you can, with few exceptions, buy tickets for any airline from any CRS. The dominant systems in the U.S. are Sabre (home to American and US Airways), Galileo (home to United), Worldspan (home to Delta, Northwest), and Amadeus (many European lines.) The company that owned Galileo and Orbitz recently bought Worldspan, so the two GDS will presumably be merged. Many of the low-price start-up airlines don't participate in any of these systems but have their own Web sites where you can check flights and buy tickets. Southwest, the largest and oldest of the low-price airlines, doesn't participate, either. Southwest's web site gets car and hotel info from Galileo, but the info seems not to flow the other way. Orbitz one of the big three online travel agencies, runs its own system which is "direct connect" linked directly to many of the airlines.
Travel dates back to antiquity where wealthy Greeks and Romans would travel for leisure to their summer homes and villas in cities such as Pompeii and Baiae.[7] While early travel tended to be slower, more dangerous, and more dominated by trade and migration, cultural and technological advances over many years have tended to mean that travel has become easier and more accessible.[8] Mankind has come a long way in transportation since Christopher Columbus sailed to the new world from Spain in 1492, an expedition which took over 10 weeks to arrive at the final destination; to the 21st century where aircraft allow travel from Spain to the United States overnight.
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