A visa is an official authorization appended to a passport, permitting entry into and travel within a particular country or region. In the U.S., the visa is an official endorsement on a passport or other document that is required to secure an alien’s admission into the U.S.
Under U.S. immigration law, an alien is any person who is not a citizen or national of the United States. There are two types of visas: nonimmigrant and immigrant. The immigration laws delineate specific categories of persons who may be eligible for an immigrant visa, which generally allows a person to live in the United States permanently and perhaps eventually seek citizenship. Persons visiting the United States on a temporary basis to engage in an activity delineated under the nonimmigrant classifications of the federal immigration laws must generally possess a nonimmigrant visa. A visit under a nonimmigrant visa may be of very short duration or may validly last for years, depending on the classification of nonimmigrant visa used.
Possession of a valid visa does not however ensure admission to the United States; an alien must still be admissible under all immigration laws at the time of arrival.