http://london.usembassy.gov/cons_new...sa_wizard.html

The visa waiver scheme allows "suitably qualified" individuals to visit the US without a full visa. All you need to do is fill in a form on the plane.

However, there are exemptions meaning that people who have been arrested may not be able to take advantage of the scheme.

It can be difficult to know exactly what that means and so the above link will take you to the official embassy visa pages where you can answer a few simple questions and get an indication of whether you can use the scheme or not.

Please note "Neither possession of a visa, nor meeting the basic requirements for traveling visa-free on the VWP, guarantees admission to the United States. As with most countries, the final determination of admissibility is made by immigration officials at the port of entry.

Visa-free travel is not appropriate for those who plan to study, work, or remain more than 90 days. Such travelers need visas. If an immigration official has reason to believe that a visa-free traveler is going to study, work, or remain longer than 90 days, or has misrepresented facts regarding his or her travel, the officer is authorized to refuse admission to the U.S."