Citizens of foreign countries, in most cases, are required to obtain a visa before entering the United States. A visa doesn’t permit entry to the U.S., however. A visa simply indicates that an application has been reviewed by a U.S. consular officer at an American embassy or consulate, and that the officer has determined the person is eligible to enter the U.S. for a specific purpose. Consular affairs are the responsibility of the U.S. Department of State.
A visa allows someone to travel to the United States as far as the port of entry (airport, seaport or land border crossing) and to ask the immigration officer to be allowed to enter the U.S. Only the immigration officer has the authority to permit entry to the United States. The immigration officer decides how long someone can stay for any particular visit. Immigration matters are the responsibility of the U.S. Department of Homeland Security.
There are two categories of U.S. visas: immigrant and nonimmigrant. Immigrant visas are for people who intend to live permanently in the U.S. Nonimmigrant visas are for people with permanent residence outside the U.S., but who wish to be in the U.S. on a temporary basis, for tourism, medical treatment, business, temporary work, or study.