Visitor Visas of The United States of America:
Generally, a citizen of a foreign country who wishes to enter another country must first obtain a visa, either a non-immigrant visa for temporary stay, or an immigrant visa for permanent residence. Visitor visas are non-immigrant visas for persons who want to enter another country temporarily for business, tourism, pleasure, short-term study, visiting family or relatives, or a combination of any of these purposes.
Different countries may have a list of countries whose citizens may be exempted from obtaining a visa and may have a facility of getting their visas on arrival (at the port of entry).
Different countries issue visitor visa for different durations, however, a visitor is typically allowed to stay in a foreign country for up to 6 months.