what is a visa?

A citizen of a foreign country who seeks to enter the United States (U.S.) generally must first obtain a U.S. visa, which is placed in the traveler’s passport, a travel document issued by the traveler’s country of citizenship. Certain international travelers may be eligible to travel to the U.S. if they meet the requirements for visa-free travel. (U.S. citizens don’t need a U.S. visa for travel, but when may need a visa issued by the country they wish to visit.)
sourse:travel.state.gov/visa/questions/questions_4429.html