Please note that it's compulsory for anyone from a country belonging to the Visa Waiver Programme who is travelling to, or changing planes in, the USA to have completed an ESTA (Electronic System for Travel Authorisation) at least 72 hours BEFORE travel. Without this passengers may be refused boarding by the airline.
The ESTA is valid for 2 years, but details must be updated for each trip you take to the USA.
For More information click here