If you are a citizen of a foreign country, in most cases you will need a visa to enter the United States.
Having a U.S. visa allows you to travel to a port of entry, airport or land border crossing, and request permission of the Department of Homeland Security (DHS), Customs and Border Protection (CBP) inspector to enter the U.S. While having a visa does not guarantee entry to the U.S, it does indicate a consular officer at a U.S Embassy or Consulate abroad has determined you are eligible to seek entry for that specific purpose. DHS/CBP inspectors, guardians of the nation’s borders, are responsible for admission of travelers to the U.S., for a specified status and period of time DHS also has responsibility for immigration matters while you are present in the U.S.
Immigrant visas are for people who intend to live permanently in the U.S.
Non-immigrant 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.