Since the beginning of 2009, if you go to travel to the United States, and you meet the conditions to not apply for a visa, you now have the option to fill out a form, known as IS, which will allow you to facilitate the passage through customs when arriving in that country.
This form, which is an alternative to the green form that can be completed on the same plane before arriving in the United States, has been fill in online at least three days before the trip, so we will get a prior authorization to travel to the United States.
In the blog GUIDE TRAVEL TO THE UNITED STATES you find all the useful information to know how complete the ESTA form, to get authorization to travel to the United States.
UPDATE 5 March 2010: The completion of ESTA form as of now it is mandatory
