The U.S. visa is a document, obtained at a U.S. Embassy or Consulate, granting a foreign traveler permission to apply for admission into the United States at a port of entry. It is not a guarantee of entry into the United States. Department of Homeland Security inspectors determine if, and for how long, a non-citizen is admitted.
For people who intend to live permanently in the United States, as provided by the Immigration and Nationality Act.
For people who wish to visit the United States temporarily–for tourism, medical treatment, business, temporary work, or study.