The Visa Waiver Program (VWP) is a U.S. government program, established in 1986, that allows people from 38 VWP-participant countries to come to the United States for business or tourism, for up to 90 days (extensions are not allowed) and without obtaining a Visa at a U.S. Embassy or Consulate .
To get it, you must obtain authorization through the Electronic System for Travel Authorization (ESTA) before entering U.S. territory. ESTA is the web-based system to establish eligibility to travel without a Visa to the U.S. for tourism (WT classification) or business (WB classification).
You must be a citizen or national of one of the 38 VWP-participant countries. Here the full list.
The travel authorization must be requested online prior to the trip. On the other hand, Form I-94W is normally completed while flying or at the U.S. port of entry. The Customs and Borders Protection has developed a system to eliminate the paper I-94W so that all the entries to the countries are recorded electronically.
One of the VWP conditions is to be in possession of a round trip ticket indicating return passage to a country outside the United States .
Your passport must be valid for at least 6 months after your planned departure from the United States (unless exempted by country-specific agreements).
Even if you are eligible for the VWP, you have to keep in mind that if you are going to change your status (for example to temporary worker) or wish to stay in the U.S. more than 90 days, you must apply for a B1 or B2 visa.
The Visa Waiver Program should not be confused with the waivers of inadmissibility, which are special permissions granted by the USCIS to individuals that are ineligible to get a Visa or Green Card due to criminal history (either from his or her home country, or in the United States) or prior immigration violations.
If your Visa Waiver Program application was denied, you should consult with an immigration appeals lawyer.