Europeans travelling to the US now have to electronically submit their details in advance. The online questionnaire hosted on a Deptartment of Homeland Security website is called ESTA (Electronic System for Travel Authorisation). It is a replacement for the diminutive green slips that you had to complete prior to landing.
Now you won’t be allowed to fly without having completed the form online and been pre-approved. This means completing the form at least 72 hours prior to departure. On the plus side once you have an approved ESTA number it is valid for two years or until your passport runs out. Whilst the site won’t win any prizes for design it is easy-to-use. All you world travellers out there need to bookmark the site.