The Visa Waiver Program (VWP) exempts nationals from 27 countries, including Portugal, from the visa requirement when traveling to the United States for tourism or business, for stays of 90 days or less. The same applies to US citizens when traveling to Portugal and other European Union countries.
To qualify to enter the USA on the grounds of VWP, the traveler must have a machine-readable or an Electronic Passport (that started to be issued by the Portuguese Government in June 2006).
The Visa Waiver Program does not cover citizens who wish to work, study or have residence in the USA. Those must apply for the adequate type of visa. A list of visas can be found at the United States Department of State webpage, namely at http://travel.state.gov/visa/visa_1750.html.
American citizens wishing to study work or apply for residence in Portugal, must also abide by visa regulations and requirements here.
It is also important to bear in mind the fact that when entering the US through the Visa Waiver Program, a visitor will not be able to change his status in US territory; he will not be able to apply for a student or work visa permit. For that he will need to go back to his country of residence and apply at the competent US Embassy or Consulate. Also, if a person fails to comply with VWP requirements (for instance by overstaying in the US after the 90 days allowed), he will not be eligible to be part in the program in the future.
Since January 12, 2009, an Electronic System of Travel Authorization, ESTA is required to travel to the US. To learn more about ESTA and to actually obtain it you can go to the United States Governments Department of Homeland Security website: https://esta.cbp.dhs.gov.
