All travellers to the United States that do not have a US passport are required to have an ESTA or a visa, issued by U.S. Customs and Border Protection. Travelling with an ESTA is only allowed for trips with a tourist or business purpose, and can last a maximum of 90 consecutive days in the US. In all other cases, a visa needs to be applied for.
To apply for an ESTA, the traveller needs to possess a valid passport. Furthermore, a number of general questions will be asked in the application form.