You are legally required to complete an ESTA (Electronic System for Travel Authorization) before travelling to the United States of America. If you have not complete an ESTA before travel you will be denied access into the country. In summary, an ESTA form allows the US government to collect information about you before your arrival. This is compulsory for everybody who wants to travel to America. It costs $14 for each ESTA application.
Make sure you complete an ESTA before booking your holiday!
Many people do not complete their ESTA until after they have booked their holiday. This can be huge mistake.
If you book a holiday before submitting your ESTA, and your ESTA application is denied (which can happen, especially if you have a criminal record) you will NOT receive a refund on your deposit from your travel agent, and your travel insurance will not reimburse you either. If the worse should happen and you cannot get your ESTA approved, you will be out a lot of money. Paying the $14 each up front to check you're safe to travel is the most sensible action you can take.
Navigate to the US Embassy website to complete your ESTA before making your Disney World booking.